Add task notifications, JSON backup, and comprehensive test suite

Notifications:
- TaskCheckWorker (WorkManager, 15-min periodic) scans cached memos for
  tasks due today or overdue, fires Android notifications
- TaskNotificationManager: notification channel, tap-to-open intent
- Notification deduplication via SharedPreferences (notified task IDs)
- Toggle in settings (on/off persisted in DataStore)
- POST_NOTIFICATIONS permission requested on API 33+

JSON Backup:
- BackupManager: export all memos to JSON, import from JSON
- Export: saves to device via Android SAF (CreateDocument)
- Import: reads JSON file, creates memos via API, refreshes cache
- Backup format: version, exportedAt, memoCount, array of BackupMemo
- Export/import buttons in settings with status feedback

Test Suite (67 tests, all passing):
- TaskParserTest (29): checkbox parsing, priorities, dates, labels, lists,
  stable IDs, toggle/replace, line indices, edge cases
- DtoMappersTest (16): ID extraction, timestamps, visibility, properties,
  attachments, reactions, comment count from relations
- DtoSerializationTest (7): JSON deserialization, unknown fields, relation
  object refs, attachment size strings
- BackupManagerTest (7): export/import round-trip, version, memo count,
  invalid JSON handling
- MemoVisibilityTest (5): fromApiString, toApiString, round-trip
- ApiResultTest (3): Success, Error, NetworkError sealed class

Co-Authored-By: Claude Opus 4.6 (1M context)

Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>

androidApp/src/main/AndroidManifest.xml

@@ -2,6 +2,7 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
 
     <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
 
     <application
         android:allowBackup="true"

androidApp/src/main/kotlin/com/avinal/memos/MainActivity.kt

@@ -1,10 +1,17 @@
 package com.avinal.memos
 
+import android.Manifest
+import android.content.pm.PackageManager
+import android.os.Build
 import android.os.Bundle
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
 import androidx.compose.runtime.CompositionLocalProvider
+import androidx.core.app.ActivityCompat
+import androidx.core.content.ContextCompat
+import com.avinal.memos.notifications.TaskNotificationManager
+import com.avinal.memos.notifications.scheduleTaskChecker
 import com.avinal.memos.util.LocalAppDependencies
 
 class MainActivity : ComponentActivity() {
@@ -20,6 +27,11 @@ class MainActivity : ComponentActivity() {
         super.onCreate(savedInstanceState)
         deps.initialize()
         com.avinal.memos.util.appContext = applicationContext
+
+        TaskNotificationManager.createChannel(this)
+        requestNotificationPermission()
+        scheduleTaskChecker(applicationContext)
+
         enableEdgeToEdge()
         setContent {
             CompositionLocalProvider(LocalAppDependencies provides deps) {
@@ -27,4 +39,14 @@ class MainActivity : ComponentActivity() {
             }
         }
     }
+
+    private fun requestNotificationPermission() {
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
+            if (ContextCompat.checkSelfPermission(this, Manifest.permission.POST_NOTIFICATIONS)
+                != PackageManager.PERMISSION_GRANTED
+            ) {
+                ActivityCompat.requestPermissions(this, arrayOf(Manifest.permission.POST_NOTIFICATIONS), 1001)
+            }
+        }
+    }
 }