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

build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    slint_build::compile("src/ui/slint/main.slint").unwrap();
+}