nwc: add downloadable/printable LNURL QR for wallet connections

Each NWC wallet connection gets its own shareable LNURL QR so the owner
can receive payments from anyone without creating invoices (the core
LNURL value proposition: a static QR that never expires).

Hub UI (service detail modal):
- New 'Share QR' button on every wallet connection card
- Dedicated share view: large QR, Lightning Address + copy, raw bech32
  LNURL + copy
- Download PNG (websites/social), download SVG (vector, print-sharp at
  any size), and a Print button that opens a print-ready payment card
  which auto-invokes the browser print dialog

Backend (FastAPI, session-authenticated):
- GET /api/nwc/wallets/{id}/lnurl -> alias, lightning_address, LNURL
- GET /api/nwc/wallets/{id}/lnurl-qr.png (?download=1, ?scale=)
- GET /api/nwc/wallets/{id}/lnurl-qr.svg (?download=1)
- GET /api/nwc/wallets/{id}/lnurl-qr/print (print-ready HTML card)
- Stdlib bech32 encoder (BIP-173, verified against the official LUD-01
  golden vector); LNURL is uppercased for QR alphanumeric-mode density
  per LUD-01 guidance; QR rendered with the existing qrencode package
- Resolves wallets by app id or pubkey; 404 unknown wallet, 503 when no
  Lightning domain is configured; all routes return 401 unauthenticated

No new system dependencies.

Co-authored-by: arena-agent <297053741+arena-agent@users.noreply.github.com>

app/sovran_systemsos_web/static/css/modals.css

@@ -517,7 +517,32 @@ button.btn-reboot:hover:not(:disabled) {
 }
 
 .nwc-wallet-actions .matrix-form-back,
-.nwc-wallet-actions .btn-close-modal {
+.nwc-wallet-actions .btn-close-modal,
+.nwc-wallet-actions .matrix-action-btn {
   padding: 8px 12px;
   font-size: 0.82rem;
 }
+
+/* ── NWC LNURL share / download view ───────────────────────────── */
+
+.nwc-share-actions {
+  display: flex;
+  gap: 8px;
+  flex-wrap: wrap;
+  margin-top: 14px;
+}
+
+.nwc-share-actions .matrix-form-back,
+.nwc-share-actions .matrix-form-submit {
+  padding: 8px 12px;
+  font-size: 0.82rem;
+}
+
+.nwc-lnurl-value {
+  font-size: 0.72rem;
+  word-break: break-all;
+}
+
+.nwc-share-formats-hint {
+  text-align: center;
+}