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/payloads/dm.rs

@@ -1,5 +1,34 @@
 use serde::{Deserialize, Serialize};
 
+#[derive(Serialize, Deserialize, Debug)]
+pub struct E2eeDmKeyExchangePayload {
+    pub dm_id: String,
+    pub e2ee_public_key: Vec<u8>,
+}
+
+#[derive(Serialize, Deserialize, Debug)]
+pub struct E2eeDmKeyExchangeAckPayload {
+    pub dm_id: String,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone)]
+pub struct E2eeDmMessagePayload {
+    pub dm_id: String,
+    pub sender_id: String,
+    #[serde(with = "serde_bytes")]
+    pub ciphertext: Vec<u8>,
+    pub timestamp: i64,
+}
+
+#[derive(Serialize, Deserialize, Debug)]
+pub struct E2eeDmHistoryPayload {
+    pub dm_id: String,
+    #[serde(default)]
+    pub messages: Vec<E2eeDmMessagePayload>,
+    #[serde(default)]
+    pub limit: Option<i64>,
+}
+
 #[derive(Serialize, Deserialize, Debug)]
 pub struct DmStartPayload {
     pub target_user_id: String,