fix(fe): auto-run the mask when the mask screen is active

The mask screen never populated its overlay: pickCalibration set overlay:null
and the only thing that computed it (approveMask) was gated behind the
Approve button, which is disabled until overlay.slabPx>0 - a chicken-and-egg,
so the preview stayed empty and Approve was permanently disabled. Auto-run
approveMask() whenever the mask screen is active (and on sheet/sensitivity
change) so the overlay and preview render immediately.

frontend/src/capture/SessionCapture.svelte

@@ -70,6 +70,13 @@
 
   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) {
+    void session.approveMask();
+  }
+
   function pickFiles(e: Event) {
     const input = e.target as HTMLInputElement;
     if (!input.files) return;
@@ -164,7 +171,9 @@
         </label>
       </div>
     {/if}
-    {#if failed}
+    {#if overlay && overlay.slabPx === 0}
+      <p class="err">No slab pixels detected — the mask removed everything. Lower the sensitivity, or set the sheet to match what the photo sits on, then retake if the shot is dark.</p>
+    {:else if failed}
       <p class="err">No slab pixels detected. Adjust controls or retake.</p>
     {:else if overlay}
       <p>Mask settled: {overlay.slabPx} slab pixels.</p>