fix(fe): debounce mask auto-run 500ms after slider/sheet changes

Rapid sensitivity drags triggered a flood of async mask runs; an in-flight
run could settle after the owner pressed Approve and clobber the state, so
the Approve click read a snapshot with overlay:null and early-returned.
Debounce 500ms after the user stops adjusting so the mask settles before any
interaction and the click reads a stable state.

frontend/src/capture/SessionCapture.svelte

@@ -71,12 +71,17 @@
   let fileInput: HTMLInputElement;
 
   // Auto-run the mask so the overlay (and preview) populate without the owner
-  // pressing Approve first. `controlKey` changes only on sheet/sensitivity
-  // edits (never on the overlay itself), so this fires on entry and on each
-  // control change - but not when approveMask settles the overlay (no loop).
+  // pressing Approve first. Debounced 500ms after the user stops adjusting the
+  // sheet/sensitivity, so rapid slider drags don't trigger a flood of async
+  // mask runs (and an in-flight run can't clobber the state around an Approve
+  // click). Keyed on control values, not the overlay, so settling never loops.
+  let maskRunTimer = 0;
   $: controlKey = kind === 'mask' && cap.control ? `${cap.control.sheet}:${cap.control.sensitivity}` : null;
   $: if (controlKey && cap.source && cap.source.widthPx > 0) {
-    void session.approveMask();
+    clearTimeout(maskRunTimer);
+    maskRunTimer = setTimeout(() => {
+      void session.approveMask();
+    }, 500);
   }
 
   function pickFiles(e: Event) {