Add server-side metrics ring buffer, seed dashboard charts from server history

Background task samples system (CPU/RAM/GPU) and per-target (FPS/timing) metrics
every 1s into a 120-sample ring buffer (~2 min). New API endpoint
GET /system/metrics-history returns the buffer. Dashboard charts now seed from
server history on load instead of sessionStorage, surviving page refreshes.

Also removes emoji from brightness source labels.

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

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

@@ -424,7 +424,7 @@
   "kc.pattern_template": "Pattern Template:",
   "kc.pattern_template.hint": "Select the rectangle pattern to use for color extraction",
   "kc.pattern_template.none": "-- Select a pattern template --",
-  "kc.brightness_vs": "🔢 Brightness Source:",
+  "kc.brightness_vs": "Brightness Source:",
   "kc.brightness_vs.hint": "Optional value source that dynamically controls brightness each frame (multiplied with the manual brightness slider)",
   "kc.brightness_vs.none": "None (manual brightness only)",
   "kc.created": "Key colors target created successfully",
@@ -810,7 +810,7 @@
   "value_source.deleted": "Value source deleted",
   "value_source.delete.confirm": "Are you sure you want to delete this value source?",
   "value_source.error.name_required": "Please enter a name",
-  "targets.brightness_vs": "🔢 Brightness Source:",
+  "targets.brightness_vs": "Brightness Source:",
   "targets.brightness_vs.hint": "Optional value source that dynamically controls brightness each frame (overrides device brightness)",
   "targets.brightness_vs.none": "None (device brightness)"
 }