Add composite layer preview, configurable LED count, and notification fire button to CSS test modal

- Composite sources show per-layer strip canvases with composite result on top
- Server sends composite wire format with per-layer RGB data
- LED count is configurable via input field, persisted in localStorage
- Notification sources show a bell fire button on the strip preview
- Composite with notification layers shows per-layer fire buttons
- Fixed stale WS frame bug with generation counter and unique consumer IDs
- Modal width is now fixed at 700px to prevent layout jumps
- Target card composite layers now use same-height canvases

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

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

@@ -931,6 +931,9 @@
   "color_strip.test.title": "Test Preview",
   "color_strip.test.connecting": "Connecting...",
   "color_strip.test.error": "Failed to connect to preview stream",
+  "color_strip.test.led_count": "LEDs:",
+  "color_strip.test.apply": "Apply",
+  "color_strip.test.composite": "Composite",
   "color_strip.type.daylight": "Daylight Cycle",
   "color_strip.type.daylight.desc": "Simulates natural daylight over 24 hours",
   "color_strip.type.daylight.hint": "Simulates the sun's color temperature throughout a 24-hour day/night cycle — from warm sunrise to cool daylight to warm sunset and dim night.",