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/templates/modals/target-editor.html

@@ -35,7 +35,7 @@
 
                 <div class="form-group">
                     <div class="label-row">
-                        <label for="target-editor-brightness-vs" data-i18n="targets.brightness_vs">🔢 Brightness Source:</label>
+                        <label for="target-editor-brightness-vs" data-i18n="targets.brightness_vs">Brightness Source:</label>
                         <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?" data-i18n-aria-label="aria.hint">?</button>
                     </div>
                     <small class="input-hint" style="display:none" data-i18n="targets.brightness_vs.hint">Optional value source that dynamically controls brightness each frame (overrides device brightness)</small>