feat(ecash): Minibits @minibits.cash Lightning address on Cashu receive

The wallet used Minibits only as a Cashu mint, so the node could hold and
swap ecash there but had no addressable name at it. This derives a LUD-16
Lightning address (name@minibits.cash) from the node's own ecash wallet and
surfaces it in the ecash Receive tab above the existing paste-token box.

Identity reuses the NUT-13 ecash phrase, so there is no second secret:
  - seedHash = sha256(mnemonic.to_seed("")) — the exact hash the Minibits app
    stores, so restoring the same phrase recovers the same address both ways;
  - Nostr keys via NIP-06 at m/44'/1237'/0'/0/0 (nostr-sdk Keys::from_mnemonic,
    pinned by a unit test against the NIP-06 vector so a bump cannot silently
    move the derivation and orphan the profile).

Backend (wallet/minibits.rs) implements the verified live /v3 flow: NIP-42
challenge/verify -> JWT, idempotent /profile registration with collision
retry, and /claim polling that NIP-04-decrypts each token (service pubkey read
from the address's own LUD-16 metadata, constant fallback) and redeems it
through ecash::receive_token. Mainnet-only; state cached 0600 in
wallet/minibits.json.

New RPC: wallet.ecash-lnaddress (register-or-read, idempotent) and
wallet.ecash-lnaddress-claim (sweep Lightning payments into ecash). The modal
fetches the address on tab open, renders QR + copy, and sweeps claims while
open; a registration failure is non-fatal so paste-token still works.

Verified end-to-end against production: registered a disposable
@minibits.cash address, confirmed it resolves via /.well-known/lnurlp, and the
claim poll returns cleanly.

neode-ui/src/locales/en.json

@@ -775,6 +775,12 @@
     "paymentConfirmed": "Payment confirmed",
     "transactionId": "Transaction ID",
     "pasteEcashToken": "Paste ecash token",
+    "lnAddressTitle": "Or share your Minibits Lightning address",
+    "lnAddressHint": "Anyone can pay you sats with any Lightning wallet by sending to this address — the sats arrive as ecash. Keep this screen open to receive them.",
+    "lnAddressLabel": "Your @minibits.cash address:",
+    "lnAddressLoading": "Setting up your Lightning address…",
+    "lnAddressUnavailable": "Lightning address unavailable — you can still paste a token below.",
+    "lnAddressReceived": "Received {amount} sats to your Lightning address!",
     "processing": "Processing...",
     "generateAddress": "Generate Address",
     "createInvoice": "Create Invoice",