Add start/stop, test, and notification buttons to graph editor node overlays

- Output targets and sync clocks get start/stop (▶/■) with optimistic UI update
- Test/preview button for templates, sources, and KC targets
- Notification test button (🔔) for notification color strip sources
- Fetch batch states to show correct running status for output targets
- Sync clocks show running state from API (is_running)
- Surgical DOM patching (patchNodeRunning) preserves hover state on toggle
- Success button hover style (green) for start action

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

server/src/wled_controller/static/css/graph-editor.css

@@ -479,6 +479,14 @@
     fill: var(--primary-contrast);
 }
 
+.graph-node-overlay-btn.success:hover rect {
+    fill: var(--success-color);
+}
+
+.graph-node-overlay-btn.success:hover text {
+    fill: var(--primary-contrast);
+}
+
 /* ── Selection rectangle ── */
 
 .graph-selection-rect {