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

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

README.md

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+# Scrap Tracer
+
+Turn a phone photo of a laser-cut scrap piece into a 1:1 DXF/SVG outline, so you can check in
+RDWorks (or LightBurn/Inkscape) whether your design fits on it. Built for Makerspace Delft.
+
+## Use
+
+1. Photograph the piece from straight above, on a contrasting surface, with something of known length visible.
+2. Load the photo, click **Measure**, click both ends of the known length, and enter it in mm.
+3. Check the green outline. If needed, adjust the threshold, tick "darker than background",
+   click the piece, or draw the outline by hand.
+4. Set the safety margin (default −2 mm shrinks the usable area a little).
+5. Download DXF and import it in your laser software at 1:1, in mm.
+
+## Develop
+
+    npm install
+    npm run dev      # local dev server
+    npm test         # unit tests (Vitest, OpenCV runs headless in Node)
+    npm run check    # type check
+    npm run build    # static build in dist/
+
+## Deploy (Cloudflare Pages)
+
+Build command `npm run build`, output directory `dist`. No server or environment variables needed.