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/tutorials.css

@@ -73,7 +73,7 @@
     background: var(--card-bg);
     border: 2px solid var(--primary-color);
     border-radius: 8px;
-    box-shadow: 0 8px 24px rgba(0, 0, 0, 0.5);
+    box-shadow: 0 8px 24px var(--shadow-color);
     z-index: 102;
     pointer-events: auto;
     animation: tutorial-tooltip-in 0.25s ease-out;
@@ -101,7 +101,7 @@
 .tutorial-close-btn {
     background: none;
     border: none;
-    color: #777;
+    color: var(--text-muted);
     font-size: 1.3rem;
     width: 24px;
     height: 24px;