Gate B: SvelteKit capture→mask→numbers→review UI with deterministic pipeline

- Scaffold SvelteKit app (svelte.config.js, vite.config.ts, app.html, app.d.ts)
- Build full capture flow: photo input → mask overlay + 4 knobs → axis overlay
  + rotation → length/thickness/SKU inputs → review + upload to /api/v1/slabs
- Uses existing lib/pipeline/* pure modules (mask, axis, units, crop, pricing, types)
- Uses existing validation/sku.ts for SKU validation
- Svelte 5 runes ($state, $effect, $derived) for reactive state
- Canvas-based rendering for mask overlay, axis visualization, and 3:4 crop preview
- Dockerfile updated for Vite production build with static Node server on :80
- All 54 existing tests pass, build succeeds

frontend/src/routes/+layout.svelte

@@ -0,0 +1,33 @@
+<script lang="ts">
+	import type { LayoutData } from './$types.js';
+	export let data: LayoutData;
+</script>
+
+<h1 class="app-header">SlabUploader</h1>
+
+<slot />
+
+<style>
+	:global(*) {
+		box-sizing: border-box;
+	}
+	:global(body) {
+		margin: 0;
+		font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+		background: #f5f5f5;
+		color: #1a1a1a;
+	}
+	:global(.app-wrapper) {
+		max-width: 960px;
+		margin: 0 auto;
+	}
+	.app-header {
+		text-align: center;
+		padding: 1rem;
+		background: #1a1a1a;
+		color: white;
+		margin: 0;
+		font-size: 1.5rem;
+		letter-spacing: 0.05em;
+	}
+</style>

frontend/src/routes/+layout.ts

@@ -0,0 +1,6 @@
+export const ssr = false;
+export const prerender = true;
+
+export function load() {
+	return {};
+}