Remove per-source speed, fix device dirty check, and add frontend caching

Speed is now exclusively controlled via sync clocks — CSS sources no longer
carry their own speed/cycle_speed fields. Streams default to 1.0× when no
clock is assigned. Also fixes false-positive dirty check on the device
settings modal (array reference comparison) and converts several frontend
modules to use DataCache for consistent API response caching.

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

server/src/wled_controller/static/sw.js

@@ -7,7 +7,7 @@
  *   - Navigation: network-first with offline fallback
  */
 
-const CACHE_NAME = 'ledgrab-v7';
+const CACHE_NAME = 'ledgrab-v8';
 
 // Only pre-cache static assets (no auth required).
 // Do NOT pre-cache '/' — it requires API key auth and would cache an error page.