Add mouse support

[aknarts]

The mouse events should be detected and supported.

Example use cases:

• Selecting/Switching rooms
• Selecting a message
• Starting a message

[sgrimee]

Investigation Findings

Current State:

• Mouse events are not handled; only keyboard input is processed
• Mouse capture is explicitly disabled to enable native terminal text selection (as noted in CHANGELOG.md)

Required Changes:
To implement mouse support for room/message selection and message composition, the following steps are needed:

1. Enable mouse capture in init() to allow mouse event detection
2. Add Mouse variant to Event enum in
3. Modify EventHandler to capture and send mouse events alongside key events
4. Add Event::Mouse handling in event loop
5. Add process_mouse_event method to App struct for handling mouse clicks
6. Implement mouse click detection for room selection in rooms list
7. Implement mouse click detection for message selection in messages list
8. Implement mouse click to start composing a new message
9. Update CHANGELOG.md to reflect that mouse capture is now enabled, disabling native text selection

Note: Enabling mouse capture will disable native terminal text selection, replacing it with custom mouse-based selection in the TUI.