Fix gallery login errors and photo upload issues

- Fix photo upload cross-device link error
  - Changed fs.rename to fs.copyFile + fs.unlink to handle Docker volume mounts
  - This fixes EXDEV errors when uploading photos from /tmp to storage

- Improve gallery login error handling
  - Add console logging for debugging
  - Prevent form refresh with stopPropagation
  - Show specific error messages based on status codes
  - Keep password field populated on error for retry
  - Map 404 to gallery not found message

- Enhanced upload error messages
  - Show specific error message for each failed file
  - Display backend error messages in toast notifications

These fixes resolve:
1. Photo uploads failing silently with cross-device link errors
2. Login errors not displaying and form refreshing
3. Upload errors not showing user-friendly messages

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

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

backend/src/services/photoProcessor.js

@@ -63,7 +63,9 @@ async function processUploadedPhotos(files, eventId, uploadedBy = 'admin', categ
       await fs.mkdir(destPath, { recursive: true });
       
       const newPath = path.join(destPath, newFilename);
-      await fs.rename(file.path, newPath);
+      // Use copyFile and unlink instead of rename to avoid cross-device issues
+      await fs.copyFile(file.path, newPath);
+      await fs.unlink(file.path);
       
       // Generate thumbnail
       const thumbnailPath = await generateThumbnail(newPath);