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

Cargo.toml

@@ -24,10 +24,14 @@ path = "src/main.rs"
 name = "vnox-e2e-voice"
 path = "src/bin/e2e_voice.rs"
 
+[build-dependencies]
+slint-build = "1"
+
 [dependencies]
 tokio              = { version = "1.52", features = ["full"] }
 serde              = { version = "1", features = ["derive"] }
 serde_json         = "1"
+serde_bytes        = "0.11"
 ed25519-dalek      = { version = "2", features = ["rand_core"] }
 x25519-dalek       = { version = "2", features = ["static_secrets"] }
 chacha20poly1305   = "0.10"
@@ -44,7 +48,7 @@ opus               = "0.3"
 cpal               = "0.18"
 rodio              = "0.22"
 dirs               = "6"
-eframe             = { version = "0.33", features = ["wgpu"] }
+slint              = { version = "1", features = ["backend-winit", "compat-1-2"] }
 nnnoiseless        = { version = "0.5.2", default-features = false, optional = true }
 
 [features]