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/geometry/rect.ts

@@ -0,0 +1,94 @@
+import type { ABox, BBox } from '../capture/capture.types';
+
+export function slabBounds(bitset: Uint8Array, w: number, h: number): ABox {
+  let minX = w, minY = h, maxX = -1, maxY = -1;
+  for (let y = 0; y < h; y++) {
+    const row = y * w;
+    for (let x = 0; x < w; x++) {
+      if (bitset[row + x]) {
+        if (x < minX) minX = x;
+        if (x > maxX) maxX = x;
+        if (y < minY) minY = y;
+        if (y > maxY) maxY = y;
+      }
+    }
+  }
+  if (maxX < 0) return { x: 0, y: 0, w: 0, h: 0 };
+  return { x: minX, y: minY, w: maxX - minX + 1, h: maxY - minY + 1 };
+}
+
+// Convex hull (Andrew's monotone chain). Points are [x, y].
+export function convexHull(pts: [number, number][]): [number, number][] {
+  const p = [...pts].sort((a, b) => a[0] - b[0] || a[1] - b[1]);
+  if (p.length < 3) return p;
+  const cross = (o: [number, number], a: [number, number], b: [number, number]) =>
+    (a[0] - o[0]) * (b[1] - o[1]) - (a[1] - o[1]) * (b[0] - o[0]);
+  const lower: [number, number][] = [];
+  for (const pt of p) {
+    while (lower.length >= 2 && cross(lower[lower.length - 2], lower[lower.length - 1], pt) <= 0) lower.pop();
+    lower.push(pt);
+  }
+  const upper: [number, number][] = [];
+  for (let i = p.length - 1; i >= 0; i--) {
+    const pt = p[i];
+    while (upper.length >= 2 && cross(upper[upper.length - 2], upper[upper.length - 1], pt) <= 0) upper.pop();
+    upper.push(pt);
+  }
+  lower.pop();
+  upper.pop();
+  return lower.concat(upper);
+}
+
+// Rotating calipers: true minimum-area bounding rectangle of the mask.
+export function minAreaRect(bitset: Uint8Array, w: number, h: number): BBox | null {
+  const pts: [number, number][] = [];
+  for (let y = 0; y < h; y++) {
+    const row = y * w;
+    for (let x = 0; x < w; x++) if (bitset[row + x]) pts.push([x, y]);
+  }
+  if (pts.length < 2) return null;
+  const hull = convexHull(pts);
+
+  let best = { area: Infinity, cx: 0, cy: 0, ux: 1, uy: 0, hw: 0, hh: 0 };
+  const n = hull.length;
+  for (let i = 0; i < n; i++) {
+    const j = (i + 1) % n;
+    const ex = hull[j][0] - hull[i][0];
+    const ey = hull[j][1] - hull[i][1];
+    const len = Math.hypot(ex, ey);
+    if (len === 0) continue;
+    const ux = ex / len, uy = ey / len;
+    const vx = -uy, vy = ux;
+    let minU = Infinity, maxU = -Infinity, minV = Infinity, maxV = -Infinity;
+    for (let k = 0; k < n; k++) {
+      const dx = hull[k][0] - hull[i][0];
+      const dy = hull[k][1] - hull[i][1];
+      const u = dx * ux + dy * uy;
+      const v = dx * vx + dy * vy;
+      if (u < minU) minU = u;
+      if (u > maxU) maxU = u;
+      if (v < minV) minV = v;
+      if (v > maxV) maxV = v;
+    }
+    const rw = maxU - minU;
+    const rh = maxV - minV;
+    const area = rw * rh;
+    if (area < best.area) {
+      best = { area, cx: hull[i][0] + (minU + maxU) / 2 * ux + (minV + maxV) / 2 * vx, cy: hull[i][1] + (minU + maxU) / 2 * uy + (minV + maxV) / 2 * vy, ux, uy, hw: rw / 2, hh: rh / 2 };
+    }
+  }
+
+  // long-side direction: the edge with the larger extent
+  const lu = best.hw >= best.hh ? best.ux : -best.uy;
+  const lv = best.hw >= best.hh ? best.uy : best.ux;
+  const su = best.hw >= best.hh ? -best.uy : best.ux;
+  const sv = best.hw >= best.hh ? -best.ux : -best.uy;
+  const halfL = best.hw >= best.hh ? best.hw : best.hh;
+  const halfS = best.hw >= best.hh ? best.hh : best.hw;
+  const corners: [number, number][] = [];
+  for (const [a, b] of [[1, 1], [1, -1], [-1, -1], [-1, 1]] as const) {
+    corners.push([best.cx + a * halfL * lu + b * halfS * su, best.cy + a * halfL * lv + b * halfS * sv]);
+  }
+  const angleDeg = ((Math.atan2(lv, lu) * 180) / Math.PI + 180) % 180;
+  return { corners, longSidePx: halfL * 2, angleDeg };
+}