feat(frontend): swap QRious for ant-design-vue's a-qrcode

- Migrate SubPage, QrPanel and TwoFactorModal from a QRious canvas to
  <a-qrcode type="svg">, which renders the QR matrix as crispEdges
  SVG rectangles — pixel-perfect at any display size or DPR, no more
  white scan-line artifacts from non-integer canvas scaling
- Drop the now-unused qrious dependency and its manualChunks entry
- Default the panel to ultra-dark on first load (existing user
  preferences in localStorage are preserved)
- Let the sub controller read subpage.html from web/dist/ first and
  fall back to the embedded copy, so Vite rebuilds in dev no longer
  require a Go recompile to refresh the asset hashes

frontend/package.json

@@ -15,9 +15,7 @@
     "ant-design-vue": "^4.2.6",
     "axios": "^1.7.9",
     "dayjs": "^1.11.20",
-    "moment": "^2.30.1",
     "otpauth": "^9.5.1",
-    "qrious": "^4.0.2",
     "qs": "^6.13.1",
     "vue": "^3.5.13",
     "vue-i18n": "^11.1.4",