feat(tui): syntax highlighting via syntect with theme picker (#11447)

## Summary

Adds syntax highlighting to the TUI for fenced code blocks in markdown
responses and file diffs, plus a `/theme` command with live preview and
persistent theme selection. Uses syntect (~250 grammars, 32 bundled
themes, ~1 MB binary cost) — the same engine behind `bat`, `delta`, and
`xi-editor`. Includes guardrails for large inputs, graceful fallback to
plain text, and SSH-aware clipboard integration for the `/copy` command.

<img width="1554" height="1014" alt="image"
src="https://github.com/user-attachments/assets/38737a79-8717-4715-b857-94cf1ba59b85"
/>

<img width="2354" height="1374" alt="image"
src="https://github.com/user-attachments/assets/25d30a00-c487-4af8-9cb6-63b0695a4be7"
/>

## Problem

Code blocks in the TUI (markdown responses and file diffs) render
without syntax highlighting, making it hard to scan code at a glance.
Users also have no way to pick a color theme that matches their terminal
aesthetic.

## Mental model

The highlighting system has three layers:

1. **Syntax engine** (`render::highlight`) -- a thin wrapper around
syntect + two-face. It owns a process-global `SyntaxSet` (~250 grammars)
and a `RwLock<Theme>` that can be swapped at runtime. All public entry
points accept `(code, lang)` and return ratatui `Span`/`Line` vectors or
`None` when the language is unrecognized or the input exceeds safety
guardrails.

2. **Rendering consumers** -- `markdown_render` feeds fenced code blocks
through the engine; `diff_render` highlights Add/Delete content as a
whole file and Update hunks per-hunk (preserving parser state across
hunk lines). Both callers fall back to plain unstyled text when the
engine returns `None`.

3. **Theme lifecycle** -- at startup the config's `tui.theme` is
resolved to a syntect `Theme` via `set_theme_override`. At runtime the
`/theme` picker calls `set_syntax_theme` to swap themes live; on cancel
it restores the snapshot taken at open. On confirm it persists `[tui]
theme = "..."` to config.toml.

## Non-goals

- Inline diff highlighting (word-level change detection within a line).
- Semantic / LSP-backed highlighting.
- Theme authoring tooling; users supply standard `.tmTheme` files.

## Tradeoffs

| Decision | Upside | Downside |
| ------------------------------------------------ |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
-----------------------------------------------------------------------------------------------------------------------
|
| syntect over tree-sitter / arborium | ~1 MB binary increase for ~250
grammars + 32 themes; battle-tested crate powering widely-used tools
(`bat`, `delta`, `xi-editor`). tree-sitter would add ~12 MB for 20-30
languages or ~35 MB for full coverage. | Regex-based; less structurally
accurate than tree-sitter for some languages (e.g. language injections
like JS-in-HTML). |
| Global `RwLock<Theme>` | Enables live `/theme` preview without
threading Theme through every call site | Lock contention risk
(mitigated: reads vastly outnumber writes, single UI thread) |
| Skip background / italic / underline from themes | Terminal BG
preserved, avoids ugly rendering on some themes | Themes that rely on
these properties lose fidelity |
| Guardrails: 512 KB / 10k lines | Prevents pathological stalls on huge
diffs or pastes | Very large files render without color |

## Architecture

```
config.toml  ─[tui.theme]─>  set_theme_override()  ─>  THEME (RwLock)
                                                              │
                  ┌───────────────────────────────────────────┘
                  │
  markdown_render ─── highlight_code_to_lines(code, lang) ─> Vec<Line>
  diff_render     ─── highlight_code_to_styled_spans(code, lang) ─> Option<Vec<Vec<Span>>>
                  │
                  │   (None ⇒ plain text fallback)
                  │
  /theme picker   ─── set_syntax_theme(theme)    // live preview swap
                  ─── current_syntax_theme()      // snapshot for cancel
                  ─── resolve_theme_by_name(name) // lookup by kebab-case
```

Key files:

- `tui/src/render/highlight.rs` -- engine, theme management, guardrails
- `tui/src/diff_render.rs` -- syntax-aware diff line wrapping
- `tui/src/theme_picker.rs` -- `/theme` command builder
- `tui/src/bottom_pane/list_selection_view.rs` -- side content panel,
callbacks
- `core/src/config/types.rs` -- `Tui::theme` field
- `core/src/config/edit.rs` -- `syntax_theme_edit()` helper

## Observability

- `tracing::warn` when a configured theme name cannot be resolved.
- `Config::startup_warnings` surfaces the same message as a TUI banner.
- `tracing::error` when persisting theme selection fails.

## Tests

- Unit tests in `highlight.rs`: language coverage, fallback behavior,
CRLF stripping, style conversion, guardrail enforcement, theme name
mapping exhaustiveness.
- Unit tests in `diff_render.rs`: snapshot gallery at multiple terminal
sizes (80x24, 94x35, 120x40), syntax-highlighted wrapping, large-diff
guardrail, rename-to-different-extension highlighting, parser state
preservation across hunk lines.
- Unit tests in `theme_picker.rs`: preview rendering (wide + narrow),
dim overlay on deletions, subtitle truncation, cancel-restore, fallback
for unavailable configured theme.
- Unit tests in `list_selection_view.rs`: side layout geometry, stacked
fallback, buffer clearing, cancel/selection-changed callbacks.
- Integration test in `lib.rs`: theme warning uses the final
(post-resume) config.

## Cargo Deny: Unmaintained Dependency Exceptions

This PR adds two `cargo deny` advisory exceptions for transitive
dependencies pulled in by `syntect v5.3.0`:

| Advisory | Crate | Status |
|----------|-------|--------|
| RUSTSEC-2024-0320 | `yaml-rust` | Unmaintained (maintainer
unreachable) |
| RUSTSEC-2025-0141 | `bincode` | Unmaintained (development ceased;
v1.3.3 considered complete) |

**Why this is safe in our usage:**

- Neither advisory describes a known security vulnerability. Both are
"unmaintained" notices only.
- `bincode` is used by syntect to deserialize pre-compiled syntax sets.
Again, these are **static vendored artifacts** baked into the binary at
build time. No user-supplied bincode data is ever deserialized. - Attack
surface is zero for both crates; exploitation would require a
supply-chain compromise of our own build artifacts.
- These exceptions can be removed when syntect migrates to `yaml-rust2`
and drops `bincode`, or when alternative crates are available upstream.

codex-rs/tui/src/lib.rs

@@ -105,6 +105,7 @@ mod streaming;
 mod style;
 mod terminal_palette;
 mod text_formatting;
+mod theme_picker;
 mod tooltips;
 mod tui;
 mod ui_consts;
@@ -545,6 +546,7 @@ async fn run_ratatui_app(
     } else {
         initial_config
     };
+
     let mut missing_session_exit = |id_str: &str, action: &str| {
         error!("Error finding conversation path: {id_str}");
         restore();
@@ -693,7 +695,7 @@ async fn run_ratatui_app(
         None => None,
     };
 
-    let config = match &session_selection {
+    let mut config = match &session_selection {
         resume_picker::SessionSelection::Resume(_) | resume_picker::SessionSelection::Fork(_) => {
             load_config_or_exit_with_fallback_cwd(
                 cli_kv_overrides.clone(),
@@ -705,6 +707,17 @@ async fn run_ratatui_app(
         }
         _ => config,
     };
+
+    // Configure syntax highlighting theme from the final config — onboarding
+    // and resume/fork can both reload config with a different tui_theme, so
+    // this must happen after the last possible reload.
+    if let Some(w) = crate::render::highlight::set_theme_override(
+        config.tui_theme.clone(),
+        find_codex_home().ok(),
+    ) {
+        config.startup_warnings.push(w);
+    }
+
     set_default_client_residency_requirement(config.enforce_residency.value());
     let active_profile = config.active_profile.clone();
     let should_show_trust_screen = should_show_trust_screen(&config);
@@ -1186,6 +1199,50 @@ trust_level = "untrusted"
         Ok(())
     }
 
+    /// Regression: theme must be configured from the *final* config.
+    ///
+    /// `run_ratatui_app` can reload config during onboarding and again
+    /// during session resume/fork.  The syntax theme override (stored in
+    /// a `OnceLock`) must use the final config's `tui_theme`, not the
+    /// initial one — otherwise users resuming a thread in a project with
+    /// a different theme get the wrong highlighting.
+    ///
+    /// We verify the invariant indirectly: `validate_theme_name` (the
+    /// pure validation core of `set_theme_override`) must be called with
+    /// the *final* config's theme, and its warning must land in the
+    /// final config's `startup_warnings`.
+    #[tokio::test]
+    async fn theme_warning_uses_final_config() -> std::io::Result<()> {
+        use crate::render::highlight::validate_theme_name;
+
+        let temp_dir = TempDir::new()?;
+
+        // initial_config has a valid theme — no warning.
+        let initial_config = build_config(&temp_dir).await?;
+        assert!(initial_config.tui_theme.is_none());
+
+        // Simulate resume/fork reload: the final config has an invalid theme.
+        let mut config = build_config(&temp_dir).await?;
+        config.tui_theme = Some("bogus-theme".into());
+
+        // Theme override must use the final config (not initial_config).
+        // This mirrors the real call site in run_ratatui_app.
+        if let Some(w) = validate_theme_name(config.tui_theme.as_deref(), Some(temp_dir.path())) {
+            config.startup_warnings.push(w);
+        }
+
+        assert_eq!(
+            config.startup_warnings.len(),
+            1,
+            "warning from final config's invalid theme should be present"
+        );
+        assert!(
+            config.startup_warnings[0].contains("bogus-theme"),
+            "warning should reference the final config's theme name"
+        );
+        Ok(())
+    }
+
     #[tokio::test]
     async fn read_session_cwd_falls_back_to_session_meta() -> std::io::Result<()> {
         let temp_dir = TempDir::new()?;