feat: Radio volume control - server-wide slider synced via SSE

- Server: inlineVolume on AudioResource, POST /api/radio/volume endpoint
- Volume persisted per guild, broadcast via SSE to all clients
- Frontend: volume slider in bottom bar with debounced API calls
- Volume icon changes based on level (muted/low/normal)

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

web/src/styles.css

@@ -859,6 +859,61 @@ html, body {
   flex-shrink: 0;
 }
 
+/* ── Volume Slider ── */
+.radio-volume {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  flex-shrink: 0;
+}
+
+.radio-volume-icon {
+  font-size: 16px;
+  width: 20px;
+  text-align: center;
+  cursor: pointer;
+}
+
+.radio-volume-slider {
+  -webkit-appearance: none;
+  appearance: none;
+  width: 100px;
+  height: 4px;
+  border-radius: 2px;
+  background: var(--bg-tertiary, #383a40);
+  outline: none;
+  cursor: pointer;
+}
+
+.radio-volume-slider::-webkit-slider-thumb {
+  -webkit-appearance: none;
+  appearance: none;
+  width: 14px;
+  height: 14px;
+  border-radius: 50%;
+  background: var(--accent, #e67e22);
+  cursor: pointer;
+  border: none;
+  box-shadow: 0 0 4px rgba(0,0,0,0.3);
+}
+
+.radio-volume-slider::-moz-range-thumb {
+  width: 14px;
+  height: 14px;
+  border-radius: 50%;
+  background: var(--accent, #e67e22);
+  cursor: pointer;
+  border: none;
+  box-shadow: 0 0 4px rgba(0,0,0,0.3);
+}
+
+.radio-volume-val {
+  font-size: 11px;
+  color: var(--text-muted);
+  min-width: 32px;
+  text-align: right;
+}
+
 /* ── Station count ── */
 .radio-counter {
   position: absolute;