feat(03-02): add staleness detection to overlay screen and error surfacing to SpeedometerScreen

Overlay screen staleness:
- Add _lastUpdateTime field to track GPS data freshness
- Add _stalenessCheckTimer for periodic staleness monitoring (1s interval)
- Update _lastUpdateTime from message.timestamp in _listenToMainAppMessages
- Start staleness monitoring in initState, cancel in dispose
- Calculate staleness opacity in build() (0.5 alpha after 3s)
- Apply staleness dimming to speed text, unit text, heading icon, and heading text
- Show "--" for speed after 10s of no data (setState in timer callback)
- Immediate snap-back when fresh data arrives (no AnimatedOpacity)

SpeedometerScreen error surfacing:
- Import permission_handler for openAppSettings
- Register error callback on OverlayProvider in initState postFrameCallback
- Implement _handleOverlayError to show snackbars for overlay errors
- Permission error: snackbar with "Settings" action button that opens app settings
- Creation failure: snackbar with error message (no action button)
- Communication degraded: snackbar with error message (no action button)
- Clean up error callback in dispose (try-catch for disposed provider)

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

lib/screens/speedometer_screen.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:provider/provider.dart';
+import 'package:permission_handler/permission_handler.dart';
 import 'dart:async';
 import 'dart:math' as math;
 import 'package:wakelock_plus/wakelock_plus.dart';
@@ -28,9 +29,10 @@ class _SpeedometerScreenState extends State<SpeedometerScreen> with WidgetsBindi
   void initState() {
     super.initState();
     WidgetsBinding.instance.addObserver(this);
-    // Delay GPS init to after first frame so context is available
+    // Delay GPS init and error callback setup to after first frame so context is available
     WidgetsBinding.instance.addPostFrameCallback((_) {
       _initializeGpsManager();
+      context.read<OverlayProvider>().onError = _handleOverlayError;
     });
     _enableWakelock();
     _ensureOverlayPermission();
@@ -42,6 +44,11 @@ class _SpeedometerScreenState extends State<SpeedometerScreen> with WidgetsBindi
     WidgetsBinding.instance.removeObserver(this);
     _backgroundHeartbeatTimer?.cancel();
     WakelockPlus.disable();
+    try {
+      context.read<OverlayProvider>().onError = null;
+    } catch (e) {
+      // Provider might already be disposed
+    }
     Logger.info('SpeedometerScreen disposed', 'Main');
     super.dispose();
   }
@@ -59,6 +66,37 @@ class _SpeedometerScreenState extends State<SpeedometerScreen> with WidgetsBindi
     }
   }
 
+  void _handleOverlayError(OverlayError error) {
+    if (!mounted) return;
+
+    String message;
+    SnackBarAction? action;
+
+    switch (error) {
+      case OverlayError.permission:
+        message = 'Overlay permission required';
+        action = SnackBarAction(
+          label: 'Settings',
+          onPressed: () => openAppSettings(),
+        );
+        break;
+      case OverlayError.creationFailed:
+        message = 'Failed to create overlay window';
+        break;
+      case OverlayError.communicationDegraded:
+        message = 'Overlay communication degraded';
+        break;
+    }
+
+    ScaffoldMessenger.of(context).showSnackBar(
+      SnackBar(
+        content: Text(message),
+        action: action,
+        duration: const Duration(seconds: 4),
+      ),
+    );
+  }
+
   Future<void> _enableWakelock() async {
     try {
       await WakelockPlus.enable();