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/modal.css

@@ -27,7 +27,7 @@
     max-height: calc(100vh - 40px);
     display: flex;
     flex-direction: column;
-    box-shadow: 0 8px 32px rgba(0, 0, 0, 0.5);
+    box-shadow: 0 8px 32px var(--shadow-color);
     animation: slideUp 0.3s ease-out;
 }
 
@@ -96,7 +96,7 @@
 .modal-close-btn {
     background: none;
     border: none;
-    color: #777;
+    color: var(--text-muted);
     font-size: 1.2rem;
     width: 32px;
     height: 32px;
@@ -123,7 +123,7 @@
 }
 
 .modal-description {
-    color: #999;
+    color: var(--text-secondary);
     margin-bottom: 20px;
     line-height: 1.6;
 }
@@ -175,7 +175,7 @@
     height: 18px;
     font-size: 0.7rem;
     line-height: 1;
-    color: var(--text-secondary, #888);
+    color: var(--text-secondary);
     cursor: pointer;
     padding: 0;
     display: inline-flex;
@@ -199,14 +199,14 @@
 .input-hint {
     display: block;
     margin: 0 0 6px 0;
-    color: #666;
+    color: var(--text-muted);
     font-size: 0.85rem;
 }
 
 .field-desc {
     display: block;
     margin: 4px 0 0 0;
-    color: #888;
+    color: var(--text-secondary);
     font-size: 0.82rem;
     font-style: italic;
 }
@@ -224,7 +224,7 @@
     display: block;
     margin-bottom: 4px;
     font-size: 0.85rem;
-    color: #aaa;
+    color: var(--text-secondary);
 }
 
 .inline-field input[type="number"] {
@@ -243,7 +243,7 @@
 .device-led-info {
     display: block;
     margin-top: 4px;
-    color: var(--text-muted, #888);
+    color: var(--text-muted);
     font-size: 0.85em;
 }
 
@@ -257,7 +257,7 @@
 .segment-index-label {
     font-size: 0.8rem;
     font-weight: 600;
-    color: #888;
+    color: var(--text-secondary);
 }
 
 .btn-icon-inline {
@@ -297,7 +297,7 @@
 
 .segment-range-fields label {
     font-size: 0.82rem;
-    color: #aaa;
+    color: var(--text-secondary);
     white-space: nowrap;
 }
 
@@ -310,7 +310,7 @@
     align-items: center;
     gap: 4px;
     font-size: 0.85rem;
-    color: #aaa;
+    color: var(--text-secondary);
     cursor: pointer;
     white-space: nowrap;
 }
@@ -320,8 +320,9 @@
 }
 
 .btn-sm {
-    font-size: 0.85rem;
+    font-size: 0.8rem;
     padding: 4px 10px;
+    min-width: auto;
 }
 
 .fps-hint {
@@ -363,7 +364,7 @@
     gap: 6px;
     font-size: 0.9rem;
     font-weight: 500;
-    color: var(--text-secondary, #888);
+    color: var(--text-secondary);
     padding: 4px 0;
     user-select: none;
 }
@@ -413,12 +414,6 @@
     font-size: 1.4rem;
 }
 
-.btn-sm {
-    padding: 4px 10px;
-    font-size: 0.8rem;
-    min-width: auto;
-}
-
 .btn-display-picker {
     width: 100%;
     padding: 10px;