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>

composeApp/build.gradle.kts

@@ -17,6 +17,8 @@ kotlin {
         compileSdk = 36
         minSdk = 26
 
+        withHostTest {}
+
         compilerOptions {
             jvmTarget.set(JvmTarget.JVM_11)
         }
@@ -68,6 +70,11 @@ kotlin {
         iosMain.dependencies {
             implementation(libs.ktor.client.darwin)
         }
+
+        commonTest.dependencies {
+            implementation(kotlin("test"))
+            implementation(libs.kotlinx.coroutines.core)
+        }
     }
 }