feat: implement settings screen with reminders, biometrics, theme, and PDF export

- Daily reminder scheduling via flutter_local_notifications (inexact, repeating at chosen time)
- Biometric lock gate on app launch with auto-prompt and manual unlock button
- Theme mode selector (System/Light/Dark) persisted to settings DB
- PDF export of full mood history shared via share_plus
- Android: POST_NOTIFICATIONS, USE_BIOMETRIC permissions; switched to FlutterFragmentActivity

README.md

@@ -54,11 +54,11 @@ lib/
 ## To-do
 
 ### Core screens
-- [ ] Bottom nav shell (home, history, insights, settings)
-- [ ] History / calendar heatmap screen
-- [ ] Day detail screen — list entries for a selected day
-- [ ] Insights screen — weekly/monthly AI summaries
-- [ ] Settings screen — reminders, biometrics, theme, data export
+- [x] Bottom nav shell (home, history, insights, settings)
+- [x] History / calendar heatmap screen
+- [x] Day detail screen — list entries for a selected day
+- [x] Insights screen — weekly/monthly AI summaries
+- [x] Settings screen — reminders, biometrics, theme, data export
 - [ ] Onboarding flow — first-launch walkthrough
 
 ### Mood logging
@@ -68,12 +68,12 @@ lib/
 
 ### Activities
 - [ ] Activity management screen — add, reorder, delete custom activities
-- [ ] Seed database with default activities on first launch
+- [x] Seed database with default activities on first launch
 
 ### AI / insights
-- [ ] Model download + progress UI
+- [x] Model setup screen — manual transfer instructions + scan button
 - [ ] Query mode — ask a free-form question about your data
-- [ ] Prompt builder — serialize mood history into LLM context
+- [x] Prompt builder — serialize mood history into LLM context
 
 ### Analytics
 - [ ] Charts screen — mood trend line, activity frequency bar chart, sleep vs mood scatter