feat(photos): pick calibration photo before Mask (#60)

Add pickCalibration state/UI over batch thumbs. Continue from batch opens
pick (including single-photo confirm). User pick enters M1 Mask on that
Source only; draft stores calibrationIndex. Removes first-photo proxy.

frontend/src/App.svelte

@@ -5,6 +5,7 @@
   import type { Capture } from './capture/capture.types';
   import Resume from './capture/Resume.svelte';
   import BatchPhotos from './capture/BatchPhotos.svelte';
+  import PickCalibration from './capture/PickCalibration.svelte';
   import MaskPanel from './capture/MaskPanel.svelte';
   import AxisPanel from './capture/AxisPanel.svelte';
   import MeasurePanel from './capture/MeasurePanel.svelte';
@@ -54,6 +55,12 @@
         onRemove={session.removeFromBatch}
         onContinue={session.continueBatch}
       />
+    {:else if cap.k === 'pickCalibration'}
+      <PickCalibration
+        photos={cap.photos}
+        onPick={session.pickCalibration}
+        onBack={session.back}
+      />
     {:else if cap.k === 'mask'}
       <MaskPanel
         state={cap}