feat: UI color theming system with live preview, presets and server sync

- RGBA color picker with live preview (debounce 50ms)
- 30 preset colors + 12 theme presets (Ocean, Forest, Sunset, Royal, etc.)
- Header image selection from configurable image library
- Export/Import theme as JSON via clipboard
- CSS variable theming across all UI elements: NitroCard headers/tabs,
  context menus, buttons (primary/dark/gray), InfoStand, toolbar,
  room tools, purse, progress bars, sliders
- All elements use var(--name, fallback) for zero visual change when default
- Smooth 0.3s CSS transitions on theme change
- Server-side persistence via WebSocket (packets 10047/10048)
- Integrated Color/Image tabs into BackgroundsView panel
- All strings use LocalizeText() for i18n support
- Settings persisted in localStorage + server sync with 1s debounce
- Added react-colorful dependency

src/css/purse/PurseView.css

@@ -22,7 +22,7 @@
 			pointer-events: all;
         }
         .borderhccontent{
-            background-color: #212131;
+            background-color: var(--ui-dark-bg, #212131);
             border-radius: 0.5rem!important;
             border: 2px solid #383853;
             height: calc(100% - 3px);
@@ -46,7 +46,7 @@
     }
 
     .nitro-purse-seasonal-currency {
-        background-color: #212131;
+        background-color: var(--ui-dark-bg, #212131);
 		background: linear-gradient(to right, #5f5f8d, transparent);
 		height: 30px;
         margin-bottom: 4px;