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/src/composables/useTheme.js

@@ -16,7 +16,7 @@ function readBool(key, fallback) {
 }
 
 const isDark = readBool(STORAGE_DARK, true);
-const isUltra = readBool(STORAGE_ULTRA, false);
+const isUltra = readBool(STORAGE_ULTRA, true);
 
 export const theme = reactive({
   isDark,