Fix multiple bugs: thumbnail generation, branding settings, categories, theme, feedback icons, upload limit, email errors

Bug fixes included:

#52 - Thumbnail Generation: Added proper parsing of settings values and validation
      of Sharp fit parameter to handle JSON-encoded strings correctly

#61 - Branding Settings Not Persisting: Added _parseBoolean helper for reliable
      boolean parsing, added hide_powered_by option for white-label support

#55 - Categories Not Applied: Fixed category update logic to properly handle
      numeric category IDs, added updated_at timestamp, improved cache invalidation

#59/#56 - Gallery Layout & Apply Theme: Set isPreviewMode=true so theme changes
      immediately propagate to parent state, hidden redundant Apply button

#58 - Feedback Icons Show When Disabled: Added feedbackEnabled check to comment
      and like buttons in MasonryGalleryLayout and GridGalleryLayout

#57 - Upload Limit 100MB: Increased body parser limit from 100MB to 500MB to
      support larger batch uploads

#54 - Wrong Error Message: Enhanced email error handling with specific error
      codes and translation keys for better user feedback

backend/server.js

@@ -324,9 +324,9 @@ async function initializeRateLimiters() {
 
 // Note: Rate limiters will be initialized after database connection
 
-// Body parsing middleware with increased limits for large uploads
-app.use(express.json({ limit: '100mb' }));
-app.use(express.urlencoded({ extended: true, limit: '100mb' }));
+// Body parsing middleware with increased limits for large batch uploads
+app.use(express.json({ limit: '500mb' }));
+app.use(express.urlencoded({ extended: true, limit: '500mb' }));
 
 // Request logging for API routes (with timestamps)
 const apiRequestLogger = (req, res, next) => {