feat: Add spinning globe background to welcome and login pages

Add animated SVG globe with rotating meridians as a subtle background
element on the welcome overlay and login pages.

Also removes unused signal-cards-mockup.html.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/css/index.css

@@ -113,6 +113,77 @@ body {
     pointer-events: none;
 }
 
+/* Spinning Globe Background */
+.globe-background {
+    position: absolute;
+    top: 50%;
+    left: 50%;
+    transform: translate(-50%, -50%);
+    width: min(80vh, 80vw);
+    height: min(80vh, 80vw);
+    pointer-events: none;
+    z-index: 0;
+}
+
+.globe-svg {
+    width: 100%;
+    height: 100%;
+    color: var(--accent-cyan);
+    opacity: 0.06;
+    animation: globeSpin 60s linear infinite;
+}
+
+[data-theme="light"] .globe-svg {
+    opacity: 0.08;
+}
+
+@keyframes globeSpin {
+    from {
+        transform: rotate(0deg);
+    }
+    to {
+        transform: rotate(360deg);
+    }
+}
+
+.globe-svg .rotating-meridians {
+    transform-origin: center;
+    animation: meridianSpin 45s linear infinite reverse;
+}
+
+@keyframes meridianSpin {
+    from {
+        transform: rotate(0deg);
+    }
+    to {
+        transform: rotate(360deg);
+    }
+}
+
+.globe-svg .meridian-1 {
+    transform-origin: center;
+    animation: meridianPulse 8s ease-in-out infinite;
+}
+
+.globe-svg .meridian-2 {
+    transform-origin: center;
+    animation: meridianPulse 8s ease-in-out infinite 2s;
+}
+
+.globe-svg .meridian-3 {
+    transform-origin: center;
+    animation: meridianPulse 8s ease-in-out infinite 4s;
+}
+
+@keyframes meridianPulse {
+    0%, 100% {
+        opacity: 0.6;
+    }
+    50% {
+        opacity: 1;
+    }
+}
+
 .welcome-container {
     width: 90%;
     max-width: 900px;