feat(signer): accept bunker:// URIs, async signer permissions, NIP-46 e2e test

- SignerManager/SignerModeScreen parse both nostrconnect:// and bunker://
  (Amber presents bunker://; signer pubkey extracted before '@')
- Signer permission surface made async (permissions, can_*, is_connection_valid)
- tests/nip46_e2e.rs: full client handshake against fake Amber over a local
  relay — NIP-44 round-trip, get_public_key identity, signed-event verification,
  vault persistence asserting no secret material for remote profiles
- prettier formatting of touched frontend files

Cargo.lock

@@ -1169,6 +1169,7 @@ dependencies = [
  "argon2",
  "async-trait",
  "base64",
+ "futures-util",
  "getrandom 0.2.17",
  "hex",
  "keyring",
@@ -1179,6 +1180,7 @@ dependencies = [
  "serde_json",
  "sha2 0.10.9",
  "tokio",
+ "tokio-tungstenite",
  "uuid",
  "zeroize",
 ]