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/AxisPanel.svelte

@@ -0,0 +1,32 @@
+<script lang="ts">
+  import type { Capture } from './capture.types';
+  import PhotoStage from './PhotoStage.svelte';
+
+  export let state: Extract<Capture, { k: 'axis' }>;
+  export let rotateAxis: () => void;
+  export let approveAxis: () => void;
+  export let back: () => void;
+</script>
+
+<section class="panel">
+  <h1>Length axis</h1>
+  <p class="hint">The arrow points along the slab length. If it does not follow the slab, flip it.</p>
+  <PhotoStage blob={state.source.blob} bbox={state.approvedMask.bbox} heading={state.heading} />
+  <div class="row">
+    <button type="button" class="ghost" on:click={back}>Back</button>
+    <button type="button" class="primary" on:click={rotateAxis}>Flip 180°</button>
+    <button type="button" class="primary" on:click={approveAxis}>Approve axis</button>
+  </div>
+</section>
+
+<style>
+  .panel { padding: 16px; color: #eee; }
+  h1 { font-size: 1.3rem; margin: 0 0 8px; }
+  .hint { color: #aaa; font-size: 0.9rem; margin: 0 0 12px; }
+  .row { display: flex; gap: 10px; margin-top: 16px; }
+  .primary {
+    flex: 1; padding: 14px; font-size: 1rem; border-radius: 10px; border: 0;
+    background: #00e5ff; color: #002; font-weight: 700;
+  }
+  .ghost { flex: 0.7; padding: 14px; border-radius: 10px; border: 1px solid #444; background: transparent; color: #ccc; }
+</style>