feat: style the UI with Oat and make the empty canvas a click-to-load drop zone

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

package-lock.json

@@ -8,6 +8,7 @@
       "name": "scrap-tracer",
       "version": "0.1.0",
       "dependencies": {
+        "@knadh/oat": "0.8.3",
         "@techstark/opencv-js": "4.12.0-release.1"
       },
       "devDependencies": {
@@ -67,6 +68,11 @@
         "@jridgewell/sourcemap-codec": "^1.4.14"
       }
     },
+    "node_modules/@knadh/oat": {
+      "version": "0.8.3",
+      "resolved": "https://registry.npmjs.org/@knadh/oat/-/oat-0.8.3.tgz",
+      "integrity": "sha512-eE07LJ6RpC21PfEbu3U6dFbTO0V6m1OcXocrYr7dwN+Ffr8sx+SqVTPwHO5/+Azv0NWEU8V7frwzBQXdx7CzLA=="
+    },
     "node_modules/@oxc-project/types": {
       "version": "0.154.0",
       "resolved": "https://registry.npmjs.org/@oxc-project/types/-/types-0.154.0.tgz",