feat(capture): ship Milestone 1 browser capture app

Why
Phone and PC need a single capture flow that ends at inventory PNG and
board-foot numbers with no backend, inference, or Woo create.

Scope
Add frontend/ Vite+Svelte capture UI with Resume, Add photo, Mask,
Length axis, and Measure. Mask runs in a worker with Auto/Green/Black
and a 2.0s sensitivity debounce. Measure computes scale, widths, sqft,
bdft, and a 3:4 alpha inventory PNG. Persist one device draft in
IndexedDB. Serve the static build from frontend/Dockerfile via nginx
(deploy/nginx-m1.conf). No FastAPI.

Verification
sudo docker build -f frontend/Dockerfile -t slabuploader-m1 .
sudo docker run -d --name slabuploader-m1 -p 0.0.0.0:18080:80 slabuploader-m1
curl http://127.0.0.1:18080/ returns 200
Playwright: synthetic green-sheet photo through Calculate shows sqft/bdft

frontend/src/capture/MaskPanel.svelte

@@ -0,0 +1,93 @@
+<script lang="ts">
+  import type { Capture, Sheet } from './capture.types';
+  import PhotoStage from './PhotoStage.svelte';
+
+  export let state: Extract<Capture, { k: 'mask' }>;
+  export let busy: boolean;
+  export let setSensitivity: (n: number) => void;
+  export let setSheet: (s: Sheet) => void;
+  export let approveMask: () => void;
+  export let retakes: () => void;
+  export let back: () => void;
+
+  const sheets: Sheet[] = ['auto', 'green', 'black'];
+  $: empty = state.overlay !== null && state.overlay.slabPx === 0;
+  $: maskW = state.overlay && state.source.widthPx > 0 ? state.source.widthPx : 0;
+  $: maskH = maskW > 0 && state.overlay ? state.overlay.bitset.length / maskW : 0;
+  $: canApprove = state.overlay !== null && state.overlay.slabPx > 0;
+</script>
+
+<section class="panel">
+  <h1>Mask</h1>
+  <div class="stage-wrap">
+    <PhotoStage
+      blob={state.source.blob}
+      maskBitset={state.overlay?.bitset ?? null}
+      {maskW}
+      {maskH}
+    />
+    {#if busy}<div class="busy">Working…</div>{/if}
+    {#if state.failed}<div class="warn">Mask pass failed. Adjust and retry, or retake the photo.</div>{/if}
+    {#if empty}<div class="warn">No slab pixels in the mask. Adjust sensitivity or retake.</div>{/if}
+  </div>
+
+  <div class="controls">
+    <div class="seg" role="group" aria-label="Sheet">
+      {#each sheets as s (s)}
+        <button type="button" class:active={state.control.sheet === s} on:click={() => setSheet(s)}>
+          {s[0].toUpperCase() + s.slice(1)}
+        </button>
+      {/each}
+    </div>
+    <div class="sens">
+      <button type="button" class="thumb" aria-label="Decrease sensitivity" on:click={() => setSensitivity(state.control.sensitivity - 1)}>−</button>
+      <input
+        type="range" min="0" max="100" step="1"
+        value={state.control.sensitivity}
+        on:input={(e) => setSensitivity(Number(e.currentTarget.value))}
+      />
+      <button type="button" class="thumb" aria-label="Increase sensitivity" on:click={() => setSensitivity(state.control.sensitivity + 1)}>+</button>
+      <span class="val">{state.control.sensitivity}</span>
+    </div>
+  </div>
+
+  <div class="row">
+    <button type="button" class="ghost" on:click={back}>Back</button>
+    <button type="button" class="ghost" on:click={retakes}>Retake</button>
+    <button type="button" class="primary" disabled={!canApprove} on:click={approveMask}>
+      Approve mask
+    </button>
+  </div>
+</section>
+
+<style>
+  .panel { padding: 16px; color: #eee; }
+  h1 { font-size: 1.3rem; margin: 0 0 12px; }
+  .stage-wrap { position: relative; }
+  .busy {
+    position: absolute; top: 8px; right: 8px; background: rgba(0,0,0,0.7);
+    color: #fff; padding: 4px 10px; border-radius: 6px; font-size: 0.85rem;
+  }
+  .warn { color: #ffb4a2; margin-top: 8px; font-size: 0.9rem; }
+  .controls { margin-top: 16px; display: flex; flex-direction: column; gap: 14px; }
+  .seg { display: flex; gap: 8px; }
+  .seg button {
+    flex: 1; padding: 12px; font-size: 1rem; border-radius: 8px;
+    border: 1px solid #444; background: #1d1d1d; color: #ccc;
+  }
+  .seg button.active { background: #00e5ff; color: #002; font-weight: 700; border-color: #00e5ff; }
+  .sens { display: flex; align-items: center; gap: 10px; }
+  .thumb {
+    width: 48px; height: 48px; border-radius: 50%; font-size: 1.6rem;
+    border: 1px solid #444; background: #1d1d1d; color: #eee; flex: none;
+  }
+  input[type='range'] { flex: 1; }
+  .val { min-width: 2.2ch; text-align: right; font-variant-numeric: tabular-nums; }
+  .row { display: flex; gap: 12px; margin-top: 20px; }
+  .primary {
+    flex: 2; padding: 14px; font-size: 1.05rem; border-radius: 10px; border: 0;
+    background: #00e5ff; color: #002; font-weight: 700;
+  }
+  .primary:disabled { background: #333; color: #777; }
+  .ghost { flex: 1; padding: 14px; border-radius: 10px; border: 1px solid #444; background: transparent; color: #ccc; }
+</style>