Add real-time translation and fix Settings language dropdown

[ZhangHanDong]

Summary

• Add real-time translation feature for the room input bar with LLM-powered translation
• Fix Settings → Preferences language dropdown rendering (was invisible due to uniform/plain type mismatch)

Real-time Translation

• Translation service module (src/room/translation.rs) with OpenAI-compatible LLM API
• Settings → Labs: configure API URL, key, model with Test Connection button
• Translate button in input bar opens language selector popup (17 languages)
• Debounce-based real-time translation (500ms) as user types
• Translation preview with Apply button to replace input text
• Global config via Mutex<Option<TranslationConfig>> for cross-widget access

Settings Language Dropdown Fix

• Root cause: Makepad DropDown shader uses two types of color declarations:
  • draw_text.color = plain value (base color for get_color shader mix chain)
  • draw_text.color_hover/focus/down = uniform() (GPU uniforms mixed in)
  • All draw_bg.* colors = uniform()
• Previous attempts (custom set_type_default() types, plain value overrides) failed because they mismatched the declaration types
• Fix: override with matching types — plain for color, uniform() for everything else

Test plan

• cargo build passes
• Settings → Labs → Translation: configure API, Test Connection works
• Room input bar: click translate icon → select language → type text → translation appears
• Click Apply → translated text replaces input
• Settings → Preferences → Language dropdown renders and functions correctly
• Changing model in Settings takes effect immediately

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