Add semi-transparent blurred tab icon as background watermark

Large SVG icon on the right side of the viewport reflects the active tab,
crossfades on tab switch. Also removes overflow:hidden from cards to fix
color picker clipping.

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

server/src/wled_controller/static/css/base.css

@@ -102,6 +102,35 @@ body.modal-open {
     background: transparent;
 }
 
+/* ── Tab indicator (background watermark) ── */
+#tab-indicator {
+    position: fixed;
+    right: -10vw;
+    top: 50%;
+    transform: translateY(-50%);
+    width: 55vw;
+    height: 55vw;
+    max-width: 600px;
+    max-height: 600px;
+    pointer-events: none;
+    z-index: -1;
+    opacity: 0;
+    filter: blur(6px);
+    color: var(--primary-color);
+    transition: opacity 0.5s ease;
+}
+#tab-indicator svg {
+    width: 100%;
+    height: 100%;
+    opacity: 0.04;
+}
+#tab-indicator.tab-indicator-visible {
+    opacity: 1;
+}
+[data-theme="light"] #tab-indicator svg {
+    opacity: 0.035;
+}
+
 .container {
     padding: 20px;
 }