FIX: WSL Paste image does not work (#6793)

## Related issues:  
- https://github.com/openai/codex/issues/3939  
- https://github.com/openai/codex/issues/2292  
- https://github.com/openai/codex/issues/7528 (After correction
https://github.com/openai/codex/pull/3990)

**Area:** `codex-cli` (image handling / clipboard & file uploads)  
**Platforms affected:** WSL (Ubuntu on Windows 10/11). No behavior
change on native Linux/macOS/Windows.

## Summary

This PR fixes image pasting and file uploads when running `codex-cli`
inside WSL. Previously, image operations failed silently or with
permission errors because paths weren't properly mapped between Windows
and WSL filesystems.

## Visual Result

<img width="1118" height="798" alt="image"
src="https://github.com/user-attachments/assets/14e10bc4-6b71-4d1f-b2a6-52c0a67dd069"
/>

## Last Rust-Cli

<img width="1175" height="859" alt="image"
src="https://github.com/user-attachments/assets/7ef41e29-9118-42c9-903c-7116d21e1751"
/>

## Root cause

The CLI assumed native Linux/Windows environments and didn't handle the
WSL↔Windows boundary:

- Used Linux paths for files that lived on the Windows host
- Missing path normalization between Windows (`C:\...`) and WSL
(`/mnt/c/...`)
- Clipboard access failed under WSL

### Why `Ctrl+V` doesn't work in WSL terminals

Most WSL terminal emulators (Windows Terminal, ConEmu, etc.) intercept
`Ctrl+V` at the terminal level to paste text from the Windows clipboard.
This keypress never reaches the CLI application itself, so our clipboard
image handler never gets triggered. Users need `Ctrl+Alt+V`.

## Changes

### WSL detection & path mapping

- Detects WSL by checking `/proc/sys/kernel/osrelease` and the
`WSL_INTEROP` env var
- Maps Windows drive paths to WSL mount paths (`C:\...` → `/mnt/c/...`)

### Clipboard fallback for WSL

- When clipboard access fails under WSL, falls back to PowerShell to
extract images from the Windows clipboard
- Saves to a temp file and maps the path back to WSL

### UI improvements

- Shows `Ctrl+Alt+V` hint on WSL (many terminals intercept plain
`Ctrl+V`)
- Better error messages for unreadable images

## Performance

- Negligible overhead. The fallback adds a single FS copy to a temp file
only when needed.
- Direct streaming remains the default.

## Files changed

- `protocol/src/lib.rs` – Added platform detection module  
- `protocol/src/models.rs` – Added WSL path mapping for local images  
- `protocol/src/platform.rs` – New module with WSL detection utilities  
- `tui/src/bottom_pane/chat_composer.rs` – Added base64 data URL support
and WSL path mapping
- `tui/src/bottom_pane/footer.rs` – WSL-aware keyboard shortcuts  
- `tui/src/clipboard_paste.rs` – PowerShell clipboard fallback

## How to reproduce the original bug (pre-fix)

1. Run `codex-cli` inside WSL2 on Windows.  
2. Paste an image from the Windows clipboard or drag an image from
`C:\...` into the terminal.
3. Observe that the image is not attached (silent failure) or an error
is logged; no artifact reaches the tool.

## How to verify the fix

1. Build this branch and run `codex-cli` inside WSL2.  
2. Paste from clipboard and drag from both Windows and WSL paths.  
3. Confirm that the image appears in the tool and the CLI shows a single
concise info line (no warning unless fallback was used).

I’m happy to adjust paths, naming, or split helpers into a separate
module if you prefer.

## How to try this branch

If you want to try this before it’s merged, you can use my Git branch:

Repository: https://github.com/Waxime64/codex.git  
Branch: `wsl-image-2`

1. Start WSL on your Windows machine.
2. Clone the repository and switch to the branch:
   ```bash
   git clone https://github.com/Waxime64/codex.git
   cd codex
   git checkout wsl-image-2
   # then go into the Rust workspace root, e.g.:
   cd codex-rs
3. Build the TUI binary:
  cargo build -p codex-tui --bin codex-tui --release
4. Install the binary:
   sudo install -m 0755 target/release/codex-tui /usr/local/bin/codex
5. From the project directory where you want to use Codex, start it
with:
   cd /path/to/your/project
   /usr/local/bin/codex

On WSL, use CTRL+ALT+V to paste an image from the Windows clipboard into
the chat.

codex-rs/tui/src/key_hint.rs

@@ -49,6 +49,10 @@ pub(crate) const fn ctrl(key: KeyCode) -> KeyBinding {
     KeyBinding::new(key, KeyModifiers::CONTROL)
 }
 
+pub(crate) const fn ctrl_alt(key: KeyCode) -> KeyBinding {
+    KeyBinding::new(key, KeyModifiers::CONTROL.union(KeyModifiers::ALT))
+}
+
 fn modifiers_to_string(modifiers: KeyModifiers) -> String {
     let mut result = String::new();
     if modifiers.contains(KeyModifiers::CONTROL) {