feat: add step sidebar with scale, detection, margin and export controls

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/App.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
   import { onMount } from 'svelte';
   import ImageCanvas from './components/ImageCanvas.svelte';
+  import Sidebar from './components/Sidebar.svelte';
   import { AppState } from './lib/app-state.svelte';
   import { loadOpenCV } from './lib/opencv-loader';
 
@@ -22,10 +23,7 @@
 
 <div class="layout">
   <aside>
-    <input type="file" accept="image/*" onchange={(e) => {
-      const file = e.currentTarget.files?.[0];
-      if (file) app.loadFile(file);
-    }} />
+    <Sidebar {app} />
   </aside>
   <main ondragover={(e) => e.preventDefault()} {ondrop}>
     {#if app.image}