Fix 23 additional lint issues to reduce from 129 to 106 issues

- Add const constructors to @immutable classes (5 issues: prefer_const_constructors_in_immutables)
- Fix nullable value casts in firesApi.dart (3 issues: cast_nullable_to_non_nullable)
- Remove redundant default argument values (2 issues: avoid_redundant_argument_values)
- Make YourLocation immutable with final fields (2 issues: avoid_equals_and_hash_code_on_mutable_classes)
- Update imports and fix formatting

Reduces lint issues from 129 to 106. APK builds successfully (146 MB).

lib/utils/secret_loader.dart

@@ -5,16 +5,16 @@ import 'package:flutter/services.dart' show rootBundle;
 /// Loads secret/config files from Flutter assets as JSON.
 /// Used to load configuration files at app startup.
 class SecretLoader {
-  final String secretPath;
-
   SecretLoader({required this.secretPath});
 
+  final String secretPath;
+
   /// Load and parse JSON from asset bundle.
   /// Returns a map of the parsed JSON content.
   Future<Map<String, dynamic>> load() {
     return rootBundle.loadStructuredData<Map<String, dynamic>>(
       secretPath,
-      (jsonStr) async => json.decode(jsonStr) as Map<String, dynamic>,
+      (String jsonStr) async => json.decode(jsonStr) as Map<String, dynamic>,
     );
   }
 }