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/measure/measure.ts

@@ -0,0 +1,75 @@
+import type { BBox, Eighths, Measures, Widths } from '../capture/capture.types';
+
+// Long-side start point per heading: heading 1 starts at corners[1] so that
+// the axis reads corners[1] -> corners[0] (see rect.ts corner order).
+export function axisEndpoints(bbox: BBox, heading: 1 | -1): { start: [number, number]; end: [number, number] } {
+  return heading === 1 ? { start: bbox.corners[1], end: bbox.corners[0] } : { start: bbox.corners[0], end: bbox.corners[1] };
+}
+
+// Sampled every 6 in along the axis; each sample is a line probe perpendicular
+// to the axis, width = max run of slab px across the probe.
+function sampleWidths(
+  bitset: Uint8Array, w: number, h: number,
+  start: [number, number], end: [number, number],
+  scale: number, lengthIn: number,
+): Eighths[] {
+  const dx = end[0] - start[0];
+  const dy = end[1] - start[1];
+  const len = Math.hypot(dx, dy) || 1;
+  const ax = dx / len, ay = dy / len;
+  const px = -ay, py = ax; // perpendicular
+  const stepPx = 6 * scale;
+  const count = Math.max(1, Math.floor(lengthIn / 6) + 1);
+  const out: Eighths[] = [];
+  const half = Math.max(2, Math.round((w + h) / 4));
+  for (let i = 0; i < count; i++) {
+    const t = (i / (count - 1 || 1)) * len;
+    const ox = start[0] + ax * t;
+    const oy = start[1] + ay * t;
+    let minP = Infinity, maxP = -Infinity;
+    for (let p = -half; p <= half; p++) {
+      const x = Math.round(ox + px * p);
+      const y = Math.round(oy + py * p);
+      if (x < 0 || y < 0 || x >= w || y >= h) continue;
+      if (bitset[y * w + x]) {
+        if (p < minP) minP = p;
+        if (p > maxP) maxP = p;
+      }
+    }
+    if (maxP < minP) continue; // no slab px on this probe
+    out.push(Math.max(0, Math.round(((maxP - minP + 1) / scale) * 8)));
+  }
+  return out;
+}
+
+export function measure(
+  bitset: Uint8Array, w: number, h: number,
+  bbox: BBox, heading: 1 | -1,
+  lengthEighths: Eighths, thicknessEighths: Eighths,
+  slabPx: number,
+): Omit<Measures, 'inventoryPng'> & { sourceBelowFloor: false } {
+  const lengthIn = lengthEighths / 8;
+  const scale = bbox.longSidePx / lengthIn;
+  const { start, end } = axisEndpoints(bbox, heading);
+  const every6In = sampleWidths(bitset, w, h, start, end, scale, lengthIn);
+  let widths: Widths;
+  if (every6In.length > 0) {
+    const min = Math.min(...every6In);
+    const max = Math.max(...every6In);
+    const avg = Math.round(every6In.reduce((a, b) => a + b, 0) / every6In.length);
+    widths = { min, max, avg, every6In };
+  } else {
+    widths = { min: 0, max: 0, avg: 0, every6In: [] };
+  }
+  const sqft = Math.round((slabPx / (scale * scale) / 144) * 100) / 100;
+  const bdft = Math.round(sqft * (thicknessEighths / 8) * 100) / 100;
+  return {
+    lengthEighths,
+    thicknessEighths,
+    scale,
+    widths,
+    sqft,
+    bdft,
+    sourceBelowFloor: false,
+  };
+}