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/browser.ts

@@ -0,0 +1,26 @@
+// Supported clients: iOS Safari, Android Chrome, PC Chrome/Edge/Firefox.
+// Gated on required API presence; anything missing → hard stop.
+export function supported(): string | null {
+  if (typeof window === 'undefined') return 'window unavailable';
+  if (typeof createImageBitmap !== 'function') return 'createImageBitmap';
+  if (typeof Worker === 'undefined') return 'Worker';
+  if (typeof indexedDB === 'undefined') return 'IndexedDB';
+  const hasOffscreen =
+    typeof OffscreenCanvas !== 'undefined' &&
+    typeof OffscreenCanvas.prototype.convertToBlob === 'function';
+  const hasCanvasBlob =
+    typeof HTMLCanvasElement !== 'undefined' &&
+    typeof HTMLCanvasElement.prototype.toBlob === 'function';
+  if (!hasOffscreen && !hasCanvasBlob) return 'canvas PNG encode';
+  const ua = navigator.userAgent;
+  const isIOS = /iPad|iPhone|iPod/.test(ua) || (navigator.platform === 'MacIntel' && navigator.maxTouchPoints > 1);
+  const isAndroid = /Android/i.test(ua);
+  const isSafari = /Safari/i.test(ua) && !/Chrome|CriOS|Edg|Firefox|FxiOS/i.test(ua);
+  const isChrome = /Chrome|CriOS/i.test(ua) && !/Edg/i.test(ua);
+  const isEdge = /Edg/i.test(ua);
+  const isFirefox = /Firefox|FxiOS/i.test(ua);
+  if (isIOS && isSafari) return null;
+  if (isAndroid && isChrome) return null;
+  if (!isIOS && !isAndroid && (isChrome || isEdge || isFirefox)) return null;
+  return 'unsupported browser';
+}