fix: extend path_provider retry budget on slow Android devices (#166) (#169)

## Summary

- Increases the retry delays in `_resolveDatabasePath()` from `[100, 300, 600]` ms (~1 s) to `[200, 500, 1000, 2000]` ms (~3.7 s).
- Adds a regression test (`test/unit/database_path_test.dart`) that verifies `initDatabasePath()` does not throw when the `path_provider` channel is unavailable.

## Root cause

On some slow Android devices (e.g. the Motorola reported in #166), the `path_provider` Pigeon channel is not ready even several seconds after `runApp()` returns. The previous back-off budget of ~1 s was not enough, causing `_resolveDatabasePath()` to exhaust all retries and throw a `PlatformException`, crashing the app with the message shown in the issue.

## Test plan

- [ ] `flutter test test/unit/database_path_test.dart` passes (new regression test)
- [ ] `flutter test test/unit/` — all 325 unit tests pass
- [ ] `flutter analyze` — no issues

Fixes #166

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/169

lib/data/db/database.dart

@@ -596,8 +596,10 @@ Future<void> initDatabasePath() async {
 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];
+  // that the engine is fully initialised, with back-off. Some slow Android
+  // devices need several seconds for the Pigeon channel to become ready
+  // (issue #166), so use a longer schedule than the initial attempt.
+  const delays = [200, 500, 1000, 2000];
   for (final ms in delays) {
     try {
       final dir = await getApplicationSupportDirectory();