Optimize deployment, fix E2E flakiness, and implement database-backed threading

- Optimize task deploy-android with marker files and source/generate tracking.
- Fix flaky Android E2E test with pumpAndSettle and safety delays.
- Implement global CrashScreen and error handlers in main.dart.
- Refactor threading to use a persistent Threads table for performance.
- Add database indexes and migration for schema v18.
- Enhance coverage gate with ghost path checks and increased coverage (82%).

lib/main.dart

@@ -1,14 +1,55 @@
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import 'package:sharedinbox/data/db/database.dart';
 import 'package:sharedinbox/di.dart';
 import 'package:sharedinbox/ui/router.dart';
+import 'package:sharedinbox/ui/screens/crash_screen.dart';
 
 void main({List<Override> overrides = const []}) async {
-  WidgetsFlutterBinding.ensureInitialized();
-  await initDatabasePath();
-  runApp(ProviderScope(overrides: overrides, child: const SharedInboxApp()));
+  unawaited(
+    runZonedGuarded(
+      () async {
+        WidgetsFlutterBinding.ensureInitialized();
+
+        // Catch errors during build (e.g. layout exceptions) and show CrashScreen.
+        ErrorWidget.builder = (details) => CrashScreen(
+              exception: details.exception,
+              stackTrace: details.stack,
+            );
+
+        // Catch framework-level errors (e.g. from gestures, timers).
+        FlutterError.onError = (details) {
+          FlutterError.presentError(details);
+          runApp(
+            CrashScreen(
+              exception: details.exception,
+              stackTrace: details.stack,
+            ),
+          );
+        };
+
+        await initDatabasePath();
+        runApp(
+          ProviderScope(
+            overrides: overrides,
+            child: const SharedInboxApp(),
+          ),
+        );
+      },
+      (error, stack) {
+        // Catch unhandled async errors.
+        runApp(
+          CrashScreen(
+            exception: error,
+            stackTrace: stack,
+          ),
+        );
+      },
+    ),
+  );
 }
 
 class SharedInboxApp extends ConsumerStatefulWidget {