diff --git a/src/App.svelte b/src/App.svelte
index 5fede75..da42345 100644
--- a/src/App.svelte
+++ b/src/App.svelte
@@ -1,3 +1,64 @@
-<script lang="ts"></script>
+<script lang="ts">
+  import { onMount } from 'svelte';
+  import ImageCanvas from './components/ImageCanvas.svelte';
+  import { AppState } from './lib/app-state.svelte';
+  import { loadOpenCV } from './lib/opencv-loader';
 
-<h1>Scrap Tracer</h1>
+  const app = new AppState();
+
+  onMount(() => {
+    loadOpenCV().then(
+      ({ cv }) => app.setCv(cv),
+      () => (app.cvError = 'Could not load image processing (OpenCV).'),
+    );
+  });
+
+  function ondrop(e: DragEvent) {
+    e.preventDefault();
+    const file = e.dataTransfer?.files[0];
+    if (file) app.loadFile(file);
+  }
+</script>
+
+<div class="layout">
+  <aside>
+    <input type="file" accept="image/*" onchange={(e) => {
+      const file = e.currentTarget.files?.[0];
+      if (file) app.loadFile(file);
+    }} />
+  </aside>
+  <main ondragover={(e) => e.preventDefault()} {ondrop}>
+    {#if app.image}
+      <ImageCanvas {app} />
+    {:else}
+      <p class="empty">Drop a photo of a scrap piece here, or use “Load photo”.</p>
+    {/if}
+  </main>
+</div>
+
+<style>
+  :global(body) {
+    margin: 0;
+    font-family: system-ui, sans-serif;
+    color: #222;
+  }
+  .layout {
+    display: grid;
+    grid-template-columns: 320px 1fr;
+    height: 100vh;
+  }
+  aside {
+    overflow-y: auto;
+    border-right: 1px solid #ddd;
+    padding: 16px;
+  }
+  main {
+    position: relative;
+    background: #2b2b2b;
+  }
+  .empty {
+    color: #bbb;
+    text-align: center;
+    margin-top: 40vh;
+  }
+</style>
diff --git a/src/components/ImageCanvas.svelte b/src/components/ImageCanvas.svelte
new file mode 100644
index 0000000..e5e18d1
--- /dev/null
+++ b/src/components/ImageCanvas.svelte
@@ -0,0 +1,175 @@
+<script lang="ts">
+  import type { AppState } from '../lib/app-state.svelte';
+  import type { Point, Shape } from '../lib/types';
+  import { fitView, screenToImage, zoomAt, type View } from '../lib/view';
+
+  let { app }: { app: AppState } = $props();
+
+  let container: HTMLDivElement;
+  let canvas: HTMLCanvasElement;
+  let size = $state({ width: 0, height: 0 });
+  let view = $state<View>({ scale: 1, offsetX: 0, offsetY: 0 });
+  let drag: { start: Point; last: Point; moved: boolean } | null = null;
+
+  const photo = $derived.by(() => {
+    const image = app.image;
+    if (!image) return null;
+    const c = document.createElement('canvas');
+    c.width = image.width;
+    c.height = image.height;
+    c.getContext('2d')!.putImageData(new ImageData(image.data, image.width, image.height), 0, 0);
+    return c;
+  });
+
+  $effect(() => {
+    const observer = new ResizeObserver(([entry]) => {
+      size = { width: Math.floor(entry.contentRect.width), height: Math.floor(entry.contentRect.height) };
+    });
+    observer.observe(container);
+    return () => observer.disconnect();
+  });
+
+  $effect(() => {
+    if (app.image && size.width > 0 && size.height > 0) {
+      view = fitView(app.image.width, app.image.height, size.width, size.height);
+    }
+  });
+
+  // Registered manually: wheel listeners must be non-passive to stop page scrolling.
+  $effect(() => {
+    const onwheel = (e: WheelEvent) => {
+      e.preventDefault();
+      view = zoomAt(view, local(e), Math.exp(-e.deltaY * 0.001));
+    };
+    canvas.addEventListener('wheel', onwheel, { passive: false });
+    return () => canvas.removeEventListener('wheel', onwheel);
+  });
+
+  $effect(() => {
+    canvas.width = size.width;
+    canvas.height = size.height;
+    const ctx = canvas.getContext('2d')!;
+    ctx.setTransform(1, 0, 0, 1, 0, 0);
+    ctx.clearRect(0, 0, canvas.width, canvas.height);
+    if (!photo) return;
+
+    ctx.setTransform(view.scale, 0, 0, view.scale, view.offsetX, view.offsetY);
+    ctx.drawImage(photo, 0, 0);
+    const px = 1 / view.scale; // one screen pixel in image units
+
+    if (app.shapePx) drawShape(ctx, app.shapePx, px);
+    if (app.mode === 'manual') {
+      for (const ring of [app.manualOuter, ...app.manualHoles]) drawPolyline(ctx, ring, true, '#ff9800', px, [6 * px, 4 * px]);
+    }
+    if (app.drawing.length > 0) drawPolyline(ctx, app.drawing, false, '#ff9800', px, []);
+    drawScale(ctx, app.scalePoints, app.lengthMm, px);
+    if (app.seed) drawSeed(ctx, app.seed, px);
+  });
+
+  function drawShape(ctx: CanvasRenderingContext2D, shape: Shape, px: number) {
+    ctx.beginPath();
+    for (const ring of [shape.outer, ...shape.holes]) traceRing(ctx, ring, true);
+    ctx.fillStyle = 'rgba(0, 200, 83, 0.25)';
+    ctx.fill('evenodd');
+    ctx.strokeStyle = '#00c853';
+    ctx.lineWidth = 2 * px;
+    ctx.setLineDash([]);
+    ctx.stroke();
+  }
+
+  function drawPolyline(ctx: CanvasRenderingContext2D, points: Point[], closed: boolean, color: string, px: number, dash: number[]) {
+    ctx.beginPath();
+    traceRing(ctx, points, closed);
+    ctx.strokeStyle = color;
+    ctx.lineWidth = 2 * px;
+    ctx.setLineDash(dash);
+    ctx.stroke();
+    ctx.setLineDash([]);
+    ctx.fillStyle = color;
+    for (const p of points) {
+      ctx.beginPath();
+      ctx.arc(p.x, p.y, 3 * px, 0, Math.PI * 2);
+      ctx.fill();
+    }
+  }
+
+  function traceRing(ctx: CanvasRenderingContext2D, ring: Point[], closed: boolean) {
+    ring.forEach((p, i) => (i === 0 ? ctx.moveTo(p.x, p.y) : ctx.lineTo(p.x, p.y)));
+    if (closed) ctx.closePath();
+  }
+
+  function drawScale(ctx: CanvasRenderingContext2D, points: Point[], lengthMm: number | null, px: number) {
+    if (points.length === 0) return;
+    drawPolyline(ctx, points, false, '#e53935', px, []);
+    if (points.length === 2 && lengthMm) {
+      const mid = { x: (points[0].x + points[1].x) / 2, y: (points[0].y + points[1].y) / 2 };
+      ctx.font = `${14 * px}px sans-serif`;
+      ctx.fillStyle = '#e53935';
+      ctx.fillText(`${lengthMm} mm`, mid.x + 6 * px, mid.y - 6 * px);
+    }
+  }
+
+  function drawSeed(ctx: CanvasRenderingContext2D, p: Point, px: number) {
+    const r = 8 * px;
+    ctx.beginPath();
+    ctx.moveTo(p.x - r, p.y);
+    ctx.lineTo(p.x + r, p.y);
+    ctx.moveTo(p.x, p.y - r);
+    ctx.lineTo(p.x, p.y + r);
+    ctx.strokeStyle = '#2962ff';
+    ctx.lineWidth = 2 * px;
+    ctx.stroke();
+  }
+
+  function local(e: PointerEvent | WheelEvent): Point {
+    const rect = canvas.getBoundingClientRect();
+    return { x: e.clientX - rect.left, y: e.clientY - rect.top };
+  }
+
+  function onpointerdown(e: PointerEvent) {
+    canvas.setPointerCapture(e.pointerId);
+    const p = local(e);
+    drag = { start: p, last: p, moved: false };
+  }
+
+  function onpointermove(e: PointerEvent) {
+    if (!drag) return;
+    const p = local(e);
+    if (Math.hypot(p.x - drag.start.x, p.y - drag.start.y) > 4) drag.moved = true;
+    if (drag.moved) {
+      view = { ...view, offsetX: view.offsetX + p.x - drag.last.x, offsetY: view.offsetY + p.y - drag.last.y };
+    }
+    drag.last = p;
+  }
+
+  function onpointerup(e: PointerEvent) {
+    if (drag && !drag.moved) app.handleImageClick(screenToImage(local(e), view));
+    drag = null;
+  }
+</script>
+
+<div class="canvas-wrap" bind:this={container}>
+  <canvas
+    bind:this={canvas}
+    class:picking={app.tool !== 'none'}
+    {onpointerdown}
+    {onpointermove}
+    {onpointerup}
+  ></canvas>
+</div>
+
+<style>
+  .canvas-wrap {
+    position: absolute;
+    inset: 0;
+    overflow: hidden;
+  }
+  canvas {
+    display: block;
+    cursor: grab;
+    touch-action: none;
+  }
+  canvas.picking {
+    cursor: crosshair;
+  }
+</style>
diff --git a/src/lib/opencv-loader.ts b/src/lib/opencv-loader.ts
index 1059268..9fcbaed 100644
--- a/src/lib/opencv-loader.ts
+++ b/src/lib/opencv-loader.ts
@@ -1,12 +1,11 @@
 export type CV = typeof import('@techstark/opencv-js');
 
+// Everything returns { cv }: the cv module is a thenable, so resolving with it directly never settles.
 let loading: Promise<{ cv: CV }> | null = null;
 
-// Wrapped in an object: the cv module is a thenable, so resolving with it directly never settles.
 export function loadOpenCV(): Promise<{ cv: CV }> {
   loading ??= (async () => {
-    const mod = (await import('@techstark/opencv-js')) as unknown as { default: CV };
-    const cv = mod.default;
+    const { cv } = typeof document === 'undefined' ? await importModule() : await loadScript();
     if (!cv.Mat) {
       await new Promise<void>((resolve) => {
         (cv as unknown as { onRuntimeInitialized: () => void }).onRuntimeInitialized = () => resolve();
@@ -16,3 +15,23 @@ export function loadOpenCV(): Promise<{ cv: CV }> {
   })();
   return loading;
 }
+
+/** Node (tests). The variable specifier keeps the bundler from emitting a second OpenCV chunk. */
+async function importModule(): Promise<{ cv: CV }> {
+  const specifier = '@techstark/opencv-js';
+  const mod = (await import(/* @vite-ignore */ specifier)) as { default: CV };
+  return { cv: mod.default };
+}
+
+// A script tag, not import(): bundler CJS interop copies `then` and the import never settles.
+async function loadScript(): Promise<{ cv: CV }> {
+  const { default: url } = await import('@techstark/opencv-js/dist/opencv.js?url');
+  await new Promise<void>((resolve, reject) => {
+    const script = document.createElement('script');
+    script.src = url;
+    script.onload = () => resolve();
+    script.onerror = () => reject(new Error('Failed to load OpenCV'));
+    document.head.append(script);
+  });
+  return { cv: (window as unknown as { cv: CV }).cv };
+}
diff --git a/src/vite-env.d.ts b/src/vite-env.d.ts
new file mode 100644
index 0000000..11f02fe
--- /dev/null
+++ b/src/vite-env.d.ts
@@ -0,0 +1 @@
+/// <reference types="vite/client" />