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/src/routes/adminSettings.js

@@ -191,7 +191,8 @@ router.put('/branding', adminAuth, async (req, res) => {
       logo_position,
       logo_display_header,
       logo_display_hero,
-      logo_display_mode
+      logo_display_mode,
+      hide_powered_by
     } = req.body;
 
     const brandingSettings = {
@@ -211,7 +212,8 @@ router.put('/branding', adminAuth, async (req, res) => {
       logo_position,
       logo_display_header,
       logo_display_hero,
-      logo_display_mode
+      logo_display_mode,
+      hide_powered_by
     };
 
     // Handle favicon deletion if empty string or null is provided