Phase 2: E2EE for DM + Slint migration + dead code cleanup

- E2EE crypto (src/e2ee.rs) with X25519 key derivation from Ed25519,
  HKDF shared secret, ChaCha20-Poly1305 encrypt/decrypt
- E2EE net layer: PID 0x0071-0x0074, JSON payloads, dispatch, session handlers
- E2EE UI: lock icons in DM header and sidebar
- Slint migration: all .slint files, bridge.rs, update.rs, main.rs
- Removed egui UI (~50 files), cleaned all dead code
- Clean cargo clippy -- -D warnings

src/net/internal/dispatch/mod.rs

@@ -30,7 +30,11 @@ pub async fn incoming(pid: u16, payload: &[u8], tx: &mpsc::Sender<NetEvent>) ->
         | PID_MESSAGE_REACTION_REMOVE
         | PID_MESSAGE_EDIT
         | PID_MESSAGE_DELETE
-        | PID_FRIEND_EVENT => dm::handle(pid, payload, tx).await?,
+        | PID_FRIEND_EVENT
+        | PID_E2EE_DM_KEY_EXCHANGE
+        | PID_E2EE_DM_KEY_EXCHANGE_ACK
+        | PID_E2EE_DM_MESSAGE
+        | PID_E2EE_DM_HISTORY => dm::handle(pid, payload, tx).await?,
 
         PID_GUILD_LIST
         | PID_GUILD_CREATE