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/base.css

@@ -14,22 +14,32 @@
 /* Dark theme (default) */
 [data-theme="dark"] {
     --bg-color: #1a1a1a;
+    --bg-secondary: #242424;
     --card-bg: #2d2d2d;
     --text-color: #e0e0e0;
+    --text-secondary: #999;
+    --text-muted: #777;
     --border-color: #404040;
     --display-badge-bg: rgba(0, 0, 0, 0.4);
     --primary-text-color: #66bb6a;
+    --success-color: #28a745;
+    --shadow-color: rgba(0, 0, 0, 0.3);
     color-scheme: dark;
 }
 
 /* Light theme */
 [data-theme="light"] {
     --bg-color: #f5f5f5;
+    --bg-secondary: #eee;
     --card-bg: #ffffff;
     --text-color: #333333;
+    --text-secondary: #666;
+    --text-muted: #999;
     --border-color: #e0e0e0;
     --display-badge-bg: rgba(255, 255, 255, 0.85);
     --primary-text-color: #3d8b40;
+    --success-color: #2e7d32;
+    --shadow-color: rgba(0, 0, 0, 0.12);
     color-scheme: light;
 }