refactor: complete UI components - implement message_bubble.rs

Finalize Priority 3 UI components refactoring (5/5 complete):

- Create message_bubble.rs (437 lines) with 3 rendering functions:
  * render_date_separator() - centered date separators
  * render_sender_header() - sender headers (incoming/outgoing)
  * render_message_bubble() - messages with forward/reply/reactions

- Simplify messages.rs by removing ~300 lines:
  * Use message_grouping::group_messages() for logic
  * Use UI components for rendering
  * Cleaner separation of concerns

- Update module exports and main.rs

All 196 tests passing (188 tests + 8 benchmarks). No regressions.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/ui/components/mod.rs

@@ -10,3 +10,4 @@ pub mod emoji_picker;
 pub use input_field::render_input_field;
 pub use chat_list_item::render_chat_list_item;
 pub use emoji_picker::render_emoji_picker;
+pub use message_bubble::{render_date_separator, render_message_bubble, render_sender_header};