feat: add Timmy Chat mobile app (Expo/React Native)

- Single-screen chat interface with Timmy's sovereign AI personality
- Text messaging with real-time AI responses via server chat API
- Voice recording and playback with waveform visualization
- Image sharing (camera + photo library) with full-screen viewer
- File attachments via document picker
- Dark arcane theme matching the Timmy Time dashboard
- Custom app icon with glowing T circuit design
- Timmy system prompt ported from dashboard prompts.py
- Unit tests for chat utilities and message types

mobile-app/constants/theme.ts

@@ -0,0 +1,12 @@
+/**
+ * Thin re-exports so consumers don't need to know about internal theme plumbing.
+ * Full implementation lives in lib/_core/theme.ts.
+ */
+export {
+  Colors,
+  Fonts,
+  SchemeColors,
+  ThemeColors,
+  type ColorScheme,
+  type ThemeColorPalette,
+} from "@/lib/_core/theme";