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.

core/archipelago/src/api/rpc/dispatcher.rs

@@ -269,6 +269,8 @@ impl RpcHandler {
             "wallet.ecash-network" => self.handle_wallet_ecash_network().await,
             "wallet.ecash-set-network" => self.handle_wallet_ecash_set_network(params).await,
             "wallet.ecash-seed-status" => self.handle_wallet_ecash_seed_status().await,
+            "wallet.ecash-lnaddress" => self.handle_wallet_ecash_lnaddress().await,
+            "wallet.ecash-lnaddress-claim" => self.handle_wallet_ecash_lnaddress_claim().await,
             "wallet.ecash-seed-reveal" => self.handle_wallet_ecash_seed_reveal(params).await,
             "wallet.ecash-restore" => self.handle_wallet_ecash_restore(params).await,
             "wallet.ecash-seed-import" => self.handle_wallet_ecash_seed_import(params).await,