Android: `theme` prop (light / dark / auto)

[LeslieOA]

Next slice of #32. Wire the `theme` prop ('light' | 'dark' | 'auto') so it actually drives Sora's color scheme on Android.

Scope

• Bundle a light TextMate theme alongside the existing `darcula.json` (dark). Use Sora's sample app's `quietlight.json` — already pre-tested against Sora's strict-Gson `TMParserJSON`.
• Extend `SoraTextMate` to register both themes at setup, and expose an `applyTheme(editor, theme)` that switches the active scheme via `ThemeRegistry.setTheme` + re-applies `TextMateColorScheme.create(...)` on the editor.
• For `theme: 'auto'`, query `Configuration.uiMode & UI_MODE_NIGHT_MASK` to pick light/dark.
• Replace the no-op `setTheme` stub in `SourceEditorViewManager.kt`.
• Update `App.android.tsx` to add a theme picker UI matching iOS's pattern (segmented control with Auto / Light / Dark, or a simpler chip).

Refs #32. After this lands, only CI + docs remain on the #32 task list.