Replace sections with tabs, add device card to grid

- Switch Devices/Displays from collapsible sections to tab layout
- Remember active tab in localStorage
- Re-render display layout when switching to Displays tab
- Replace floating "+" button with dashed add-device card in grid
- Rename sections to "Devices" and "Displays"

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

server/src/wled_controller/static/style.css

@@ -168,6 +168,41 @@ section {
     grid-column: 1 / -1;
 }
 
+.add-device-card {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    cursor: pointer;
+    border: 2px dashed var(--border-color);
+    background: transparent;
+    min-height: 160px;
+    transition: border-color 0.2s, background 0.2s;
+}
+
+.add-device-card:hover {
+    border-color: var(--primary-color);
+    background: rgba(33, 150, 243, 0.05);
+}
+
+.add-device-icon {
+    font-size: 2.5rem;
+    font-weight: 300;
+    color: var(--text-secondary);
+    line-height: 1;
+    transition: color 0.2s;
+}
+
+.add-device-card:hover .add-device-icon {
+    color: var(--primary-color);
+}
+
+.add-device-label {
+    font-size: 0.85rem;
+    color: var(--text-secondary);
+    margin-top: 8px;
+}
+
 .card {
     background: var(--card-bg);
     border: 1px solid var(--border-color);
@@ -374,40 +409,43 @@ section {
     font-size: 1.2rem;
 }
 
-/* Collapsible sections */
-.collapsible-header {
+/* Tabs */
+.tab-bar {
+    display: flex;
+    align-items: center;
+    gap: 4px;
+    border-bottom: 2px solid var(--border-color);
+    margin-bottom: 16px;
+}
+
+.tab-btn {
+    background: none;
+    border: none;
+    padding: 10px 18px;
+    font-size: 1rem;
+    font-weight: 500;
+    color: var(--text-secondary);
     cursor: pointer;
-    user-select: none;
+    border-bottom: 2px solid transparent;
+    margin-bottom: -2px;
+    transition: color 0.2s, border-color 0.2s;
 }
 
-.collapsible-header:hover {
-    opacity: 0.8;
+.tab-btn:hover {
+    color: var(--text-color);
 }
 
-.collapsible-header .collapse-chevron {
-    display: inline-block;
-    font-size: 0.6em;
-    margin-left: 6px;
-    transition: transform 0.2s ease;
-    vertical-align: middle;
+.tab-btn.active {
+    color: var(--primary-color);
+    border-bottom-color: var(--primary-color);
 }
 
-.collapsible-header.collapsed .collapse-chevron {
-    transform: rotate(-90deg);
+.tab-panel {
+    display: none;
 }
 
-.collapsible-content {
-    overflow: hidden;
-    transition: max-height 0.3s ease, opacity 0.2s ease;
-    max-height: 2000px;
-    opacity: 1;
-}
-
-.collapsible-content.collapsed {
-    max-height: 0;
-    opacity: 0;
-    margin: 0;
-    padding: 0;
+.tab-panel.active {
+    display: block;
 }
 
 /* Display Layout Visualization */