Add donation banner with QR code popup

Footer with tagline, BTC address, and hover-triggered QR code
generated client-side via the qrcode library.

ui/package.json

@@ -17,5 +17,8 @@
     "svelte-check": "^4.0.0",
     "typescript": "^5.6.0",
     "vite": "^6.0.0"
+  },
+  "dependencies": {
+    "qrcode": "^1.5.4"
   }
 }