feat: Add CSS template system with custom gallery styling support

## Changes

### CSS Template System
- Added CSS class hooks to gallery components for custom template targeting
- Gallery sidebar, header, footer, and photo cards can now be styled via CSS templates
- CSS variables on :root allow themes to override colors, effects, and spacing

### Gallery Component CSS Classes Added
- `.gallery-page` - Main gallery container
- `.gallery-header` - Top header bar
- `.gallery-sidebar` - Filter/download sidebar
- `.gallery-sidebar-header`, `.gallery-sidebar-title`, `.gallery-sidebar-close`
- `.gallery-sidebar-content`, `.gallery-sidebar-section`
- `.gallery-sidebar-search-input`, `.gallery-sidebar-search-icon`
- `.gallery-sidebar-backdrop` - Mobile overlay
- `.gallery-btn`, `.gallery-btn-download` - Sidebar buttons
- `.gallery-footer` - Footer section
- `.photo-card`, `.photo-grid` - Photo display elements

### CSS Templates (Database)
- Elegant Dark (id=1): Dark navy theme with light text and red accents
- Liquid Glass Light (id=2): iOS 26 frosted glass effect with gradient background

### Bug Fixes
- Fixed CSS variables not inheriting (moved from .gallery-page to :root)
- Fixed sidebar position breaking layout (removed position: relative override)
- Fixed Elegant Dark sidebar text visibility (white on white issue)

### Other Changes
- Settings page refactoring and cleanup
- i18n locale updates for new gallery features
- Vite proxy port configuration fix
- Admin auth route improvements
- CSS templates service updates

frontend/src/services/cssTemplates.service.ts

@@ -76,7 +76,14 @@ class CssTemplatesService {
   async getGalleryCss(slug: string): Promise<string | null> {
     try {
       const response = await api.get(`/gallery/${slug}/css-template`, {
-        responseType: 'text'
+        responseType: 'text',
+        headers: {
+          'Cache-Control': 'no-cache',
+          'Pragma': 'no-cache'
+        },
+        params: {
+          _t: Date.now() // Cache-busting parameter
+        }
       });
       if (response.status === 204) {
         return null;