feat(tui): add theme config setting, suppress underline from themes

Add `[tui] theme = "..."` config option that lets users override the
auto-detected syntax highlighting theme with any of the 32 bundled
two-face themes by kebab-case name. Invalid names log a warning and
fall back to adaptive detection.

Also suppress FontStyle::UNDERLINE in convert_style — themes like
Dracula mark type scopes with underline which produces distracting
underlines on type/module names in terminal output.

codex-rs/core/config.schema.json

@@ -1400,6 +1400,11 @@
             "type": "string"
           },
           "type": "array"
+        },
+        "theme": {
+          "default": null,
+          "description": "Syntax highlighting theme name (kebab-case).\n\nWhen set, overrides automatic light/dark theme detection. Run with `--list-themes` or see docs for available names.",
+          "type": "string"
         }
       },
       "type": "object"