fix: add MatrixSdkDatabase — timeline was empty without event storage

Without a databaseBuilder, the Matrix SDK has no event storage. The
initial sync populates room metadata but timeline events are not
retained in any queryable form. room.getTimeline() returns empty.

Added MatrixSdkDatabase('m8chat_matrix') which uses IndexedDB on web
and SQLite on mobile via Hive. Events are now persisted and timeline
loads room history correctly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/core/network/matrix_client.dart

@@ -1,4 +1,4 @@
-// Version: 1.0.1 | Created: 2026-04-01
+// Version: 1.1.0 | Created: 2026-04-01 | Updated: 2026-04-03
 // Matrix SDK client singleton provider.
 // The Matrix client is kept alive for the full app lifetime once initialised.
 
@@ -14,9 +14,17 @@ part 'matrix_client.g.dart';
 /// keepAlive: true — the Matrix client must never be disposed while the app
 /// is running; it holds the sync loop and all room state.
 ///
-/// Phase 1: no databaseBuilder — uses in-memory store only (no persistence
+/// Uses [MatrixSdkDatabase] for event storage — without a database, the SDK
-/// across restarts). Phase 2 will wire in a drift-backed DatabaseApi.
+/// doesn't store timeline events and room.getTimeline() returns empty.
+/// On web this uses IndexedDB via Hive; on mobile it uses SQLite.
 @Riverpod(keepAlive: true)
 Client matrixClient(Ref ref) {
-  return Client(AppConfig.appName);
+  return Client(
+    AppConfig.appName,
+    databaseBuilder: (client) async {
+      final db = MatrixSdkDatabase('m8chat_matrix');
+      await db.open();
+      return db;
+    },
+  );
 }