feat: rebrand app and packaging icons to new Motionity logo

Swap the small icon mark, README banner, packaged app icon, and
favicon over to the new brand assets. make-icon.cjs and the new
make-favicon.cjs re-derive their bar geometry/colours from
src/assets/logo.svg instead of the old white-bars mark.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -11,6 +11,7 @@
   "scripts": {
     "vendor": "node scripts/vendor.mjs",
     "icons": "node scripts/make-icon.cjs",
+    "favicon": "node scripts/make-favicon.cjs",
     "start": "node scripts/server.cjs",
     "dev": "electron .",
     "dist:win": "npm run vendor && npm run icons && electron-builder --win",