⚡ Async font loading and virtualized font dropdown to eliminate UI stutter

[guiyanakuang]

Closes #4003

Summary

• FontManager: Replace eager availableFonts/selectableFonts with MutableStateFlow, load fonts asynchronously on Dispatchers.IO in init block. currentFontInfo uses combine() to react when fonts finish loading.
• FontSettingItemView: Replace DropdownMenu (composes all 100+ items at once) with Popup + LazyColumn (virtualized, only visible items composed). Shows loading spinner while fonts load. Auto-scrolls to selected font.
• LanguageSettingItemView: Move DropdownMenu into trailingContent with wrapContentSize(Alignment.TopEnd) for correct anchor positioning on constrained-width layouts (iPad).

Three blocking points eliminated

1. getSystemFonts() + GraphicsEnvironment.availableFontFamilyNames — moved to IO thread
2. FontFamily object creation for each system font — moved to IO thread
3. Rendering 100+ font items simultaneously — replaced with virtualized LazyColumn

Test plan

• Verify font dropdown opens without stutter
• Verify loading spinner appears briefly if fonts haven't loaded yet
• Verify font list auto-scrolls to currently selected font
• Verify selecting a font applies it correctly
• Verify language dropdown positions correctly on constrained-width layouts
• Verify app startup is not blocked by font enumeration