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/layout.css

@@ -87,7 +87,7 @@ h2 {
 }
 
 .health-dot.health-online {
-    background-color: #4CAF50;
+    background-color: var(--primary-color);
     box-shadow: 0 0 6px rgba(76, 175, 80, 0.6);
 }
 
@@ -97,7 +97,7 @@ h2 {
 }
 
 .health-dot.health-unknown {
-    background-color: #9E9E9E;
+    background-color: var(--text-secondary);
     animation: pulse 2s infinite;
 }
 
@@ -166,12 +166,32 @@ h2 {
 
 .tab-panel {
     display: none;
+    position: relative;
 }
 
 .tab-panel.active {
     display: block;
 }
 
+/* Thin animated bar at top of tab panel during data refresh */
+.tab-panel.refreshing::before {
+    content: '';
+    position: absolute;
+    top: 0;
+    left: 0;
+    width: 40%;
+    height: 2px;
+    background: var(--primary-color);
+    border-radius: 1px;
+    animation: tab-loading-slide 1.2s ease-in-out infinite;
+    z-index: 2;
+}
+
+@keyframes tab-loading-slide {
+    0%   { left: -40%; }
+    100% { left: 100%; }
+}
+
 /* Theme Toggle */
 .search-toggle,
 .theme-toggle {
@@ -247,7 +267,7 @@ h2 {
     background: var(--card-bg);
     border: 1px solid var(--border-color);
     border-radius: 12px;
-    box-shadow: 0 16px 48px rgba(0, 0, 0, 0.3);
+    box-shadow: 0 16px 48px var(--shadow-color);
     display: flex;
     flex-direction: column;
     overflow: hidden;
@@ -347,10 +367,32 @@ h2 {
     text-align: center;
 }
 
+@media (max-width: 900px) {
+    .server-info {
+        flex-wrap: wrap;
+        gap: 4px;
+    }
+}
+
 @media (max-width: 768px) {
     header {
         flex-direction: column;
         gap: 8px;
         text-align: center;
     }
+
+    .container {
+        padding: 12px;
+    }
+}
+
+@media (max-width: 600px) {
+    .tab-bar {
+        flex-wrap: wrap;
+    }
+
+    .tab-btn {
+        padding: 8px 12px;
+        font-size: 0.9rem;
+    }
 }