Multi-identity infrastructure: IdentityManager + Tauri AppState refactor

New identity.rs: IdentityManager manages per-identity data directories
under identities/{node_id_hex}/. Supports create, list, switch, delete,
import from key. Legacy flat layout auto-migrates on first launch.

Tauri AppState refactored from Arc<Node> to AppNode (Arc<RwLock<Arc<Node>>>)
+ AppIdentity (Arc<Mutex<IdentityManager>>). All 76 IPC commands updated
to get_node(&state).await pattern. Enables hot-swap identity switching.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/core/src/lib.rs

@@ -4,6 +4,7 @@ pub mod connection;
 pub mod content;
 pub mod crypto;
 pub mod http;
+pub mod identity;
 pub mod network;
 pub mod node;
 pub mod protocol;