Major frontend and admin improvements

Frontend changes:
- Removed brand filter and column from table
- Removed inventory summary grid
- Removed stanje (state) and težina (weight) columns
- Reorganized filters: Material → Finish → Color
- Updated EnhancedFilters component with new filter structure
- Removed legend section for storage conditions

Admin dashboard changes:
- Removed sidebar navigation (Boje option)
- Made dashboard full screen
- Removed brand column from table
- Removed brand field from add/edit form
- Updated all boolean columns to use checkmark/X icons
- Made color tiles 40% bigger
- Added sortable columns functionality
- Auto-fill price: 3499 for refill, 3999 for regular

Other improvements:
- Added BackToTop button component on all pages
- Fixed Next.js dialog backdrop CSS error
- Updated tests to match new UI structure
- Removed obsolete FilamentTable.tsx component
- Ensured proper synchronization between admin and frontend

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

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

src/styles/index.css

@@ -2,6 +2,14 @@
 @tailwind components;
 @tailwind utilities;
 
+/* Next.js error overlay styles */
+[data-nextjs-dialog-backdrop] {
+  position: fixed;
+  inset: 0;
+  background-color: rgba(0, 0, 0, 0.5);
+  z-index: 9999;
+}
+
 /* Prevent white flash on admin pages */
 @layer base {
   html {