feat(tui): enable syntax highlighting by default and refactor highlighting logic

- Set `default_syntax_highlighting` to true in core config.
- Added language‑aware syntax selector (`select_syntax_for_language`) and highlighter builder (`build_highlighter_for_language`) with unit test.
- Integrated new highlight module into `ChatApp`, using `UnicodeSegmentation` for proper grapheme handling.
- Simplified `should_highlight_code` to always return true and removed extended‑color detection logic.
- Reworked code rendering to use `inline_code_spans_from_text` and `wrap_highlight_segments` for accurate line wrapping and styling.
- Cleaned up removed legacy keyword/comment parsing and extended‑color detection code.

crates/owlen-core/src/config.rs

@@ -1389,7 +1389,7 @@ impl UiSettings {
     }
 
     const fn default_syntax_highlighting() -> bool {
-        false
+        true
     }
 
     const fn default_show_timestamps() -> bool {