feat: add IME (Input Method Editor) support

[Touma-Kazusa2]

Add support for IME composition input (Chinese, Japanese, Korean, etc.):

• Add TextInputImeState component to track preedit composition state
• Add ImePreedit and ImeCommit action types
• Add ime_focus_system to enable/disable IME and position candidate box
• Add ime_event_system to route Bevy Ime messages to text inputs
• Add ime_cleanup_on_unfocus_system to cancel preedit on focus loss
• Suppress keyboard input during IME composition to prevent double input
• Render preedit underline beneath composing text (color follows TextColor by default, configurable via TextInputStyle)
• Use selection-based preedit removal for robustness