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>

schemas/node.capnp

@@ -83,6 +83,14 @@ interface NodeService {
   # Create a 1:1 channel between the caller and the given peer. Returns a 16-byte channelId (UUID).
   # Both members can enqueue/fetch for this channel; recipientKey must be the other member.
   createChannel @18 (peerKey :Data, auth :Auth) -> (channelId :Data);
+
+  # Resolve a username to its Ed25519 identity key (32 bytes).
+  # Returns empty Data if the username is not registered.
+  resolveUser @19 (username :Text, auth :Auth) -> (identityKey :Data);
+
+  # Reverse lookup: resolve an Ed25519 identity key to the registered username.
+  # Returns empty Text if the identity key is not associated with any user.
+  resolveIdentity @20 (identityKey :Data, auth :Auth) -> (username :Text);
 }
 
 struct Auth {