chore(frontend): fetch latest PingEvent timestamp in repository for sync operations

- Added `selectLatestPingEventTimestamp` query to `AppDatabase`.
- Updated `PingEventRepositoryImpl` to use the timestamp for `since` parameter instead of the event ID.

frontend/core/local-db/src/commonMain/sqldelight/at/mocode/frontend/core/localdb/AppDatabase.sq

@@ -36,6 +36,12 @@ FROM PingEvent
 ORDER BY id DESC
 LIMIT 1;
 
+selectLatestPingEventTimestamp:
+SELECT last_modified
+FROM PingEvent
+ORDER BY last_modified DESC
+LIMIT 1;
+
 upsertPingEvents:
 -- SQLite dialect configured for this project is 3.18 (no UPSERT support).
 -- Use INSERT OR REPLACE as pragmatic upsert.