fix(fe): mask auto-run keyed on control, no settle loop

Key the auto-run off a controlKey (sheet:sensitivity) so it fires on mask
entry and on each sheet/sensitivity edit, but never re-fires when approveMask
settles the overlay - preventing both the empty-preview bug and an infinite
settle loop.

frontend/src/capture/SessionCapture.svelte

@@ -70,10 +70,12 @@
 
   let fileInput: HTMLInputElement;
 
-  // Auto-run the mask when entering the mask screen and whenever the sheet or
-  // sensitivity changes, so the overlay (and preview) is populated without the
-  // owner having to press Approve first.
-  $: if (kind === 'mask' && cap.source && cap.source.widthPx > 0) {
+  // 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).
+  $: controlKey = kind === 'mask' && cap.control ? `${cap.control.sheet}:${cap.control.sensitivity}` : null;
+  $: if (controlKey && cap.source && cap.source.widthPx > 0) {
     void session.approveMask();
   }