Fix linting issues and code quality improvements

Resolve 62 linting issues identified by flutter analyze, reducing
total issues from 79 to 17. All critical warnings addressed.

Changes:
- Replace deprecated withOpacity() with withValues(alpha:) (25 instances)
- Remove unused imports from 9 files
- Remove unused variables and fields (6 instances)
- Fix Riverpod 3.0 state access violations in settings_page
- Remove unnecessary null-aware operators in navigation_page (6 instances)
- Fix unnecessary type casts in providers (4 instances)
- Remove unused methods: _getDarkMapStyle, _showPermissionDialog
- Simplify hover state management by removing unused _isHovered fields

Remaining 17 issues are info-level style suggestions and defensive
programming patterns that don't impact functionality.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

lib/services/location_permission_service.dart

@@ -1,8 +1,6 @@
 import 'package:permission_handler/permission_handler.dart';
 
 class LocationPermissionService {
-  static const String _tcKey = 'navigation_tc_accepted';
-
   Future<LocationPermissionResult> requestLocationPermission() async {
     final status = await Permission.location.request();