feat(images): surface actionable upload errors for private photo pipeline

- Change uploadCarImage to throw explicit errors instead of returning null
- Handle upload exceptions in garage detail flow with clearer user feedback
- Guide users to retry with a smaller/clearer image when upload constraints fail

lib/screens/garage_screen.dart

@@ -678,14 +678,18 @@ class GarageScreenState extends State<GarageScreen> {
     showGlobalSnackBar('Uploading photo…');
 
     final oldPath = car['user_image_url'] as String?;
-    final newPath = await StorageService.uploadCarImage(
-      file: File(xFile.path),
-      entryId: car['id'] as int,
-      oldPath: oldPath,
-    );
-
-    if (newPath == null) {
-      showGlobalSnackBar('Failed to upload photo.', isError: true);
+    String newPath;
+    try {
+      newPath = await StorageService.uploadCarImage(
+        file: File(xFile.path),
+        entryId: car['id'] as int,
+        oldPath: oldPath,
+      );
+    } catch (e) {
+      showGlobalSnackBar(
+        'Failed to upload photo. Please try a smaller/clearer image. ($e)',
+        isError: true,
+      );
       return;
     }
 

lib/services/storage_service.dart

@@ -18,34 +18,29 @@ class StorageService {
 
   /// Upload a car image for a specific hotwheels entry.
   /// Returns the storage path on success (e.g. `uid/123.jpg`).
-  static Future<String?> uploadCarImage({
+  static Future<String> uploadCarImage({
     required File file,
     required int entryId,
     String? oldPath,
   }) async {
-    try {
-      if (oldPath != null && oldPath.isNotEmpty) {
-        await deleteCarImage(oldPath);
-      }
-
-      final userId = supabase.auth.currentUser!.id;
-      final path = '$userId/$entryId.jpg';
-      final compressed = await _compressImage(file);
-
-      await supabase.storage.from(_bucket).uploadBinary(
-            path,
-            compressed,
-            fileOptions: const FileOptions(
-              upsert: true,
-              contentType: 'image/jpeg',
-            ),
-          );
-
-      return path;
-    } catch (e) {
-      debugPrint('StorageService.uploadCarImage error: $e');
-      return null;
+    if (oldPath != null && oldPath.isNotEmpty) {
+      await deleteCarImage(oldPath);
     }
+
+    final userId = supabase.auth.currentUser!.id;
+    final path = '$userId/$entryId.jpg';
+    final compressed = await _compressImage(file);
+
+    await supabase.storage.from(_bucket).uploadBinary(
+          path,
+          compressed,
+          fileOptions: const FileOptions(
+            upsert: true,
+            contentType: 'image/jpeg',
+          ),
+        );
+
+    return path;
   }
 
   /// Generates a temporary signed URL for a private image path.