feat: interactive REPL with auto-setup, auto-join, encrypted local storage

REPL auto-setup (zero-friction startup):
- OnceLock → RwLock for CLIENT_AUTH to allow delayed init after OPAQUE login
- Extract opaque_register/opaque_login helpers from one-shot commands
- Token cache (.session file) with QPCE encryption when password provided
- Add --username/--password/--state-password to repl subcommand
- resolve_access_token: auto-register + login, cache token, prompt interactively
- rpassword for secure password input (no echo)

Interactive REPL (multi-conversation):
- SessionState: identity, hybrid key, ConversationStore, per-conversation GroupMembers
- ConversationStore: SQLite-backed conversations + messages with full CRUD
- Slash commands: /dm, /group, /invite, /join, /switch, /list, /members, /history, /whoami
- Background polling (1s interval) with auto-join from MLS Welcome messages
- pending_member pattern: persistent keystore for HPKE init key, replenish after join
- Self-DM handled as local-only notepad (no MLS/server channel)
- ANSI display module for colored prompts, incoming messages, status/error output

Username resolution:
- resolveIdentity RPC (@20 in node.capnp): look up username by identity key
- Server: resolve_identity_key in Store trait, FileBackedStore, SqlStore
- Client: resolve_identity in rpc.rs, used in auto-join for peer display names
- resolveUser: bidirectional lookup (username → identity key)

Encrypted local storage (nothing in cleartext):
- ConversationStore uses SQLCipher when --state-password is provided
- Argon2id key derivation with per-database random salt (.convdb-salt, mode 0600)
- Transparent migration of existing unencrypted databases via sqlcipher_export
- Token cache encrypted with QPCE format (Argon2id + ChaCha20Poly1305)

Server changes:
- resolveIdentity + resolveUser RPC handlers with auth + validation
- Auth: sealed-sender identity binding on enqueue, channel member authorization
- Delivery: hybrid decrypt attempts, identity key validation on enqueue
- Config: --allow-sealed-sender flag for anonymous delivery mode
- zeroize added to server dependencies

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -3834,6 +3834,8 @@ dependencies = [
  "quinn",
  "quinn-proto",
  "rand 0.8.5",
+ "rpassword",
+ "rusqlite",
  "rustls",
  "serde",
  "serde_json",
@@ -3928,6 +3930,7 @@ dependencies = [
  "toml 0.8.23",
  "tracing",
  "tracing-subscriber",
+ "zeroize",
 ]
 
 [[package]]
@@ -4287,6 +4290,16 @@ dependencies = [
  "windows-sys 0.52.0",
 ]
 
+[[package]]
+name = "rpassword"
+version = "5.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ffc936cf8a7ea60c58f030fd36a612a48f440610214dc54bc36431f9ea0c3efb"
+dependencies = [
+ "libc",
+ "winapi",
+]
+
 [[package]]
 name = "rusqlite"
 version = "0.31.0"