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/room/InfoStand.css

@@ -27,7 +27,7 @@
   width: clamp(160px, 20vw, 190px); /* Responsive width */
   z-index: 30;
   pointer-events: auto;
-  background: #212131;
+  background: var(--ui-dark-bg, #212131);
   box-shadow: inset 0 5px rgba(38, 38, 57, 0.6), inset 0 -4px rgba(25, 25, 37, 0.6);
   border-radius: 0.5rem;
   padding: 10px;

src/css/room/RoomWidgets.css

@@ -4,7 +4,7 @@
     left: 15px;
 
     .nitro-room-tools {
-        background: #212131;
+        background: var(--ui-dark-bg, #212131);
         box-shadow: inset 0px 5px lighten(rgba(#000, .6), 2.5), inset 0 -4px darken(rgba(#000, .6), 4);
         border-top-right-radius: .25rem;
         border-bottom-right-radius: .25rem;
@@ -54,7 +54,7 @@
     }
 
     .nitro-room-history {
-        background: #212131;
+        background: var(--ui-dark-bg, #212131);
         box-shadow: inset 0px 5px lighten(rgba(#000, .6), 2.5), inset 0 -4px darken(rgba(#000, .6), 4);
         transition: all .2s ease;
         width: 150px;
@@ -63,7 +63,7 @@
     }
 
     .nitro-room-tools-info {
-        background: #212131;
+        background: var(--ui-dark-bg, #212131);
         box-shadow: inset 0px 5px lighten(rgba(#000, .6), 2.5), inset 0 -4px darken(rgba(#000, .6), 4);
         transition: all .2s ease;
         max-width: 250px;