fix(fe): make capture photo ids unique per slab

slab_photos.id is a global primary key, but photo ids were cap-<i>-<size> -
identical for the same photo set on a different slab, so the upload hit
UNIQUE constraint failed: slab_photos.id and returned 500. Key the id on the
slab id so it is unique across slabs.

frontend/src/App.svelte

@@ -124,8 +124,10 @@
         // processedBlob so the server upload carries real images (PUBLISH-40).
         const calIdx = s.calibrationIndex;
         const apply = results?.batchApply;
+        // slab_photos.id is a global primary key, so the id must be unique
+        // across slabs - key it on the slab id, not just the index/size.
         const photos = s.batch.map((source, i) => ({
-          id: `cap-${i}-${source.blob.size}`,
+          id: `cap-${d.id}-${i}`,
           seq: i + 1,
           isCalibration: i === calIdx,
           url: URL.createObjectURL(source.blob),
@@ -144,7 +146,7 @@
         // encode failed (known separate defect) — the lineup just shows no overlay.
         if (results?.overlayPng && calIdx != null) {
           const overlay = {
-            id: `overlay-${calIdx}-${results.overlayPng.size}`,
+            id: `overlay-${d.id}-${calIdx}`,
             seq: calIdx + 2,
             isCalibration: false,
             url: URL.createObjectURL(results.overlayPng),