Frontend polish: loading states, CSS variables, focus indicators, scroll lock

- Add tab refresh loading bar animation for all 4 tab loaders
- Add profiles loading guard to prevent concurrent fetches
- Centralize theme colors into CSS variables (--text-secondary, --text-muted,
  --bg-secondary, --success-color, --shadow-color) for both dark/light themes
- Replace hardcoded gray values across 10 CSS files with variables
- Fix duplicate .btn-sm definition in modal.css
- Fix z-index: toast 2001→2500 to safely clear modals at 2000
- Add :focus-visible keyboard navigation indicators for all interactive elements
- Add responsive breakpoints for tab bar and header on narrow screens
- Prevent background page scroll when command palette is open

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/css/dashboard.css

@@ -183,7 +183,7 @@
 }
 
 .dashboard-status-dot.active {
-    color: #4CAF50;
+    color: var(--primary-color);
     animation: pulse 2s infinite;
 }
 
@@ -209,7 +209,7 @@
     border-radius: 10px;
     font-size: 0.7rem;
     font-weight: 600;
-    background: var(--success-color, #28a745);
+    background: var(--success-color);
     color: #fff;
     flex-shrink: 0;
 }