Add configurable FPS to test preview and fix composite stream release race

- Add FPS control (1-60, default 20) to test preview modal next to LED count
- Server accepts fps query param, controls frame send interval
- Single Apply icon button (✓) applies both LED count and FPS
- FPS control stays visible for picture sources (LED count hidden)
- Fix composite sub-stream consumer ID collision: use unique instance ID
  to prevent old WebSocket release from killing new connection's streams

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

server/src/wled_controller/static/locales/en.json

@@ -932,6 +932,7 @@
   "color_strip.test.connecting": "Connecting...",
   "color_strip.test.error": "Failed to connect to preview stream",
   "color_strip.test.led_count": "LEDs:",
+  "color_strip.test.fps": "FPS:",
   "color_strip.test.apply": "Apply",
   "color_strip.test.composite": "Composite",
   "color_strip.type.daylight": "Daylight Cycle",