Add NIP-46 remote signer (external signing)

Add a remote-signer (bunker) role so other Nostr apps can delegate
signing to this app's active profile keys via nostrconnect:// links.

Backend: new src/signer.rs implementing the NIP-46 protocol (kind 24133
events encrypted with NIP-44 v2 conversation keys). It parses
nostrconnect:// connect URIs, spawns an async task in the serve process
that reads relay requests, auto-approves once the handshake completes,
signs delegate events, and publishes responses. Exposes status, connect,
and disconnect via IPC and CLI (signer status / signer connect).

Other: ipc.rs serve/handle now share Arc<Mutex<App>>; main.rs adds the
signer CLI commands; Cargo.toml enables nostr nip46 feature.

Frontend: new Signer screen (nav item + sidebar entry with key icon)
to paste a nostrconnect:// link, connect/disconnect, and show the
connected peer and relays; wires signer_connect/_disconnect/_status
through api.ts and AppProvider; adds tests and test mocks.

Cargo.lock

@@ -751,6 +751,7 @@ version = "0.40.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2f900ddcdc28395759fcd44b18a03255e7deee8858551bfe5d5d5a07311d82ea"
 dependencies = [
+ "aes",
  "base64",
  "bech32",
  "bip39",
@@ -789,6 +790,7 @@ dependencies = [
  "base64",
  "getrandom 0.2.17",
  "hex",
+ "nostr",
  "nostr-sdk",
  "rpassword",
  "serde",