fix: retry path_provider on PlatformException at database open (#153, #157)

On some Android versions the path_provider Pigeon channel
('dev.flutter.pigeon.path_provider_android.PathProviderApi.getApplicationSupportPath')
is not ready when initDatabasePath() runs before runApp().  The existing code
already catches PlatformException there, leaving _dbPath null — but the
LazyDatabase callback called getApplicationSupportDirectory() a second time
without any protection, causing an unhandled crash on those devices.

Fix: extract _resolveDatabasePath() which retries three times with back-off
(100 ms → 300 ms → 600 ms) before re-throwing with a descriptive error
message. By the time the database is first accessed (after runApp()), the
channel is almost always available; if it still isn't, the CrashScreen is
shown with a clear explanation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/data/db/database.dart

@@ -591,15 +591,32 @@ Future<void> initDatabasePath() async {
   }
 }
 
+/// Resolve the application support path, retrying on PlatformException to
+/// survive a race where the path_provider Pigeon channel isn't ready yet.
+Future<String> _resolveDatabasePath() async {
+  if (_dbPath != null) return _dbPath!;
+  // initDatabasePath() failed (channel not ready before runApp). Retry now
+  // that the engine is fully initialised, with brief back-off.
+  const delays = [100, 300, 600];
+  for (final ms in delays) {
+    try {
+      final dir = await getApplicationSupportDirectory();
+      _dbPath = p.join(dir.path, 'sharedinbox.db');
+      return _dbPath!;
+    } on PlatformException {
+      await Future<void>.delayed(Duration(milliseconds: ms));
+    }
+  }
+  throw PlatformException(
+    code: 'channel-error',
+    message: 'path_provider unavailable after ${delays.length + 1} attempts — '
+        'cannot open database.',
+  );
+}
+
 LazyDatabase _openConnection() {
   return LazyDatabase(() async {
-    final file = File(
-      _dbPath ??
-          p.join(
-            (await getApplicationSupportDirectory()).path,
-            'sharedinbox.db',
-          ),
-    );
+    final file = File(await _resolveDatabasePath());
     return NativeDatabase.createInBackground(
       file,
       setup: (db) {