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/capture/PickCalibration.svelte

@@ -0,0 +1,66 @@
+<script lang="ts">
+  import { onDestroy } from 'svelte';
+  import type { Source } from './capture.types';
+
+  export let photos: Source[];
+  export let onPick: (index: number) => void;
+  export let onBack: () => void;
+
+  let urls: string[] = [];
+
+  $: {
+    for (const u of urls) URL.revokeObjectURL(u);
+    urls = photos.map((p) => URL.createObjectURL(p.blob));
+  }
+
+  onDestroy(() => {
+    for (const u of urls) URL.revokeObjectURL(u);
+  });
+</script>
+
+<section class="panel">
+  <h1>Pick calibration photo</h1>
+  <p class="hint">
+    Choose which photo is the top-down reference for length and thickness.
+    Mask, length axis, and Measure run on that photo only.
+  </p>
+
+  <ul class="thumbs" aria-label="Calibration candidates">
+    {#each photos as _p, i (urls[i])}
+      <li class="thumb">
+        <img src={urls[i]} alt={`Photo ${i + 1}`} />
+        <button
+          type="button"
+          class="pick"
+          aria-label={`Use photo ${i + 1} as calibration`}
+          on:click={() => onPick(i)}
+        >Use as calibration</button>
+      </li>
+    {/each}
+  </ul>
+
+  <button class="back" type="button" on:click={onBack}>Back</button>
+</section>
+
+<style>
+  .panel { padding: 24px 16px; color: #eee; }
+  h1 { font-size: 1.4rem; }
+  .hint { color: #bbb; line-height: 1.5; font-size: 0.95rem; }
+  .thumbs {
+    list-style: none; padding: 0; margin: 16px 0;
+    display: grid; grid-template-columns: repeat(auto-fill, minmax(140px, 1fr)); gap: 12px;
+  }
+  .thumb {
+    background: #1e1e1e; border-radius: 8px; overflow: hidden;
+    display: flex; flex-direction: column;
+  }
+  .thumb img { width: 100%; aspect-ratio: 3 / 4; object-fit: cover; display: block; background: #000; }
+  .pick {
+    border: 0; background: #00e5ff; color: #002; padding: 10px 8px;
+    font-size: 0.9rem; font-weight: 700; cursor: pointer;
+  }
+  .back {
+    margin-top: 8px; width: 100%; padding: 12px; font-size: 1rem; border-radius: 10px;
+    border: 1px solid #444; background: transparent; color: #ccc; cursor: pointer;
+  }
+</style>