feat(photos): multi-photo batch + device draft (#59)

Replace single Add photo with Batch: accumulate 1–N photos, Continue
when ≥1 (M1 bridge: mask on first), persist batch blobs in the device
draft, Resume Continue restores. Keeps Mask/Axis/Measure intact.

frontend/src/App.svelte

@@ -4,7 +4,7 @@
   import { createCaptureSession, type CaptureSession } from './capture/session';
   import type { Capture } from './capture/capture.types';
   import Resume from './capture/Resume.svelte';
-  import AddPhoto from './capture/AddPhoto.svelte';
+  import BatchPhotos from './capture/BatchPhotos.svelte';
   import MaskPanel from './capture/MaskPanel.svelte';
   import AxisPanel from './capture/AxisPanel.svelte';
   import MeasurePanel from './capture/MeasurePanel.svelte';
@@ -12,7 +12,7 @@
   const gate = typeof window !== 'undefined' ? supported() : 'window unavailable';
   const session: CaptureSession = createCaptureSession();
 
-  let cap: Capture = { k: 'addPhoto' };
+  let cap: Capture = { k: 'batch', photos: [] };
   let busy = false;
   let pngUrl: string | null = null;
 
@@ -24,8 +24,6 @@
     busy = b;
   });
 
-  const pick = (f: File) => void session.addPhoto(f);
-
   onMount(() => {
     void session.resumeProbe();
   });
@@ -49,8 +47,13 @@
   <main class="app">
     {#if cap.k === 'resume'}
       <Resume onContinue={() => session.continueDraft()} onFresh={() => void session.startFresh()} />
-    {:else if cap.k === 'addPhoto'}
-      <AddPhoto onPicked={pick} />
+    {:else if cap.k === 'batch'}
+      <BatchPhotos
+        photos={cap.photos}
+        onAdd={(f) => void session.addToBatch(f)}
+        onRemove={session.removeFromBatch}
+        onContinue={session.continueBatch}
+      />
     {:else if cap.k === 'mask'}
       <MaskPanel
         state={cap}