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

android/app/src/main/AndroidManifest.xml

@@ -1,4 +1,8 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
+    <uses-permission android:name="android.permission.USE_BIOMETRIC" />
+    <uses-permission android:name="android.permission.USE_FINGERPRINT" />
+    <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
     <application
         android:label="dailyou"
         android:name="${applicationName}"

android/app/src/main/kotlin/net/stefwill/dailyou/MainActivity.kt

@@ -1,5 +1,5 @@
 package net.stefwill.dailyou
 
-import io.flutter.embedding.android.FlutterActivity
+import io.flutter.embedding.android.FlutterFragmentActivity
 
-class MainActivity : FlutterActivity()
+class MainActivity : FlutterFragmentActivity()