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/MeasurePanel.svelte

@@ -0,0 +1,101 @@
+<script lang="ts">
+  import type { Capture } from './capture.types';
+  import { fmtEighths } from './session';
+  import PhotoStage from './PhotoStage.svelte';
+
+  export let state: Extract<Capture, { k: 'measure' }>;
+  export let setLength: (e: number) => void;
+  export let setThickness: (e: number) => void;
+  export let calculate: () => Promise<void>;
+  export let back: () => void;
+  export let pngUrl: string | null;
+
+  let calcBusy = false;
+  const doCalc = async () => {
+    calcBusy = true;
+    try { await calculate(); } finally { calcBusy = false; }
+  };
+</script>
+
+<section class="panel">
+  <h1>Measure</h1>
+  <PhotoStage blob={state.source.blob} bbox={state.axis.bbox} heading={state.axis.heading} />
+
+  <div class="fields">
+    <div class="field">
+      <span class="label">Length L</span>
+      <div class="stepper">
+        <button type="button" class="thumb" on:click={() => setLength(state.lengthEighths - 1)} aria-label="Length minus">−</button>
+        <span class="v">{fmtEighths(state.lengthEighths)}</span>
+        <button type="button" class="thumb" on:click={() => setLength(state.lengthEighths + 1)} aria-label="Length plus">+</button>
+      </div>
+    </div>
+    <div class="field">
+      <span class="label">Thickness</span>
+      <div class="stepper">
+        <button type="button" class="thumb" on:click={() => setThickness(state.thicknessEighths - 1)} aria-label="Thickness minus">−</button>
+        <span class="v">{fmtEighths(state.thicknessEighths)}</span>
+        <button type="button" class="thumb" on:click={() => setThickness(state.thicknessEighths + 1)} aria-label="Thickness plus">+</button>
+      </div>
+    </div>
+  </div>
+
+  <div class="row">
+    <button type="button" class="ghost" on:click={back}>Back</button>
+    <button
+      type="button"
+      class="primary"
+      disabled={calcBusy || state.lengthEighths === 0 || state.thicknessEighths === 0}
+      on:click={doCalc}
+    >
+      {calcBusy ? 'Calculating…' : 'Calculate'}
+    </button>
+  </div>
+
+  {#if state.results}
+    <div class="results">
+      <div class="nums">
+        <div><span>Widths</span><span>{fmtEighths(state.results.widths.min)} – {fmtEighths(state.results.widths.max)} (avg {fmtEighths(state.results.widths.avg)})</span></div>
+        <div><span>Sqft</span><span>{state.results.sqft.toFixed(2)}</span></div>
+        <div><span>Bdft</span><span>{state.results.bdft.toFixed(2)}</span></div>
+        <div><span>Scale</span><span>{state.results.scale.toFixed(2)} px/in</span></div>
+      </div>
+      {#if state.results.sourceBelowFloor}
+        <div class="warn">Source is below the 1600px floor for the inventory PNG. Retake a larger photo.</div>
+      {/if}
+      {#if pngUrl && !state.results.sourceBelowFloor}
+        <div class="png-wrap">
+          <img src={pngUrl} alt="Inventory PNG" />
+        </div>
+      {/if}
+    </div>
+  {/if}
+</section>
+
+<style>
+  .panel { padding: 16px; color: #eee; }
+  h1 { font-size: 1.3rem; margin: 0 0 12px; }
+  .fields { display: flex; gap: 16px; margin-top: 16px; }
+  .field { flex: 1; }
+  .label { display: block; font-size: 0.9rem; color: #aaa; margin-bottom: 8px; }
+  .stepper { display: flex; align-items: center; gap: 12px; }
+  .thumb {
+    width: 52px; height: 52px; border-radius: 50%; font-size: 1.7rem;
+    border: 1px solid #444; background: #1d1d1d; color: #eee; flex: none;
+  }
+  .v { flex: 1; text-align: center; font-size: 1.15rem; font-variant-numeric: tabular-nums; }
+  .row { display: flex; gap: 10px; 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; }
+  .results { margin-top: 20px; }
+  .nums { display: flex; flex-direction: column; gap: 8px; }
+  .nums div { display: flex; justify-content: space-between; border-bottom: 1px solid #2a2a2a; padding-bottom: 6px; }
+  .nums span:first-child { color: #aaa; }
+  .warn { color: #ffb4a2; margin-top: 12px; font-size: 0.9rem; }
+  .png-wrap { margin-top: 16px; text-align: center; background-image: linear-gradient(45deg,#ccc 25%,transparent 25%),linear-gradient(-45deg,#ccc 25%,transparent 25%),linear-gradient(45deg,transparent 75%,#ccc 75%),linear-gradient(-45deg,transparent 75%,#ccc 75%); background-size: 16px 16px; background-position: 0 0,0 8px,8px -8px,-8px 0; background-color: #eee; border-radius: 8px; padding: 8px; }
+  .png-wrap img { max-width: 100%; border-radius: 4px; }
+</style>