Phase 2: PostgreSQL, E2EE, channel CRUD, voice signaling, Slint migration

Server:
- PostgreSQL backend (Pool enum over Sqlite/PgPool, all storage methods dual-dialect)
- E2EE for DM (4 new protobuf packets, handlers, storage of ciphertext only)
- ChannelEdit packet + server-authoritative channel create/delete
- Gateway → voice-node membership signaling via broadcast channel

Client:
- E2EE crypto (X25519 from Ed25519, HKDF, ChaCha20-Poly1305) + UI lock icons
- E2EE net layer (PID 0x0071-0x0074, JSON payloads, dispatch, session handlers)
- Slint migration: all .slint files, bridge.rs, update.rs, main.rs rewritten
- Removed egui UI (~50 files) and all dead code (clean clippy -D warnings)

gateway/src/net/state.rs

@@ -34,6 +34,8 @@ pub struct State {
     pub rate_limiter: Arc<RateLimiter>,
     /// Gateway → voice-node membership bridge.
     pub voice_member_tx: Option<VoiceMemberTx>,
+    /// E2EE key store: user_id -> X25519 public key bytes
+    pub e2ee_keys: Arc<RwLock<HashMap<String, Vec<u8>>>>,
 }
 
 impl State {
@@ -66,6 +68,7 @@ impl State {
             channels_count,
             rate_limiter,
             voice_member_tx,
+            e2ee_keys: Arc::new(RwLock::new(HashMap::new())),
         }
     }
 }