Add dynamic WebGL background with audio reactivity

- WebGL shader background with flowing waves, radial pulse, and frequency ring arcs
- Reacts to captured audio data (frequency bands + bass) when visualizer is active
- Uses page accent color; adapts to dark/light theme via bg-primary blending
- Toggle button in header toolbar, state persisted in localStorage
- Cached uniform locations and color values to avoid per-frame getComputedStyle calls
- i18n support for EN/RU locales

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

media_server/static/css/styles.css

@@ -63,6 +63,27 @@
     box-shadow: 0 -2px 8px rgba(0, 0, 0, 0.08);
 }
 
+/* Dynamic Background Canvas */
+.bg-shader-canvas {
+    position: fixed;
+    top: 0;
+    left: 0;
+    width: 100vw;
+    height: 100vh;
+    z-index: -1;
+    pointer-events: none;
+    opacity: 0;
+    transition: opacity 0.6s ease;
+}
+
+.bg-shader-canvas.visible {
+    opacity: 1;
+}
+
+body.dynamic-bg-active {
+    background: transparent;
+}
+
 * {
     margin: 0;
     padding: 0;
@@ -218,6 +239,10 @@ h1 {
     background: var(--bg-tertiary);
 }
 
+.header-btn.active {
+    color: var(--accent);
+}
+
 .header-btn svg {
     width: 16px;
     height: 16px;
@@ -3165,7 +3190,6 @@ footer .separator {
     font-weight: 500;
     text-align: center;
     transition: transform 0.3s ease;
-    animation: bannerSlideIn 0.4s ease-out;
 }
 
 .connection-banner:not(.hidden) {