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/Cargo.toml

@@ -36,7 +36,7 @@ hex.workspace                = true
 
 axum  = { version = "0.8", features = ["ws"] }
 tower = "0.5"
-sqlx     = { version = "0.9", features = ["sqlite", "runtime-tokio", "tls-rustls"] }
+sqlx     = { version = "0.9", features = ["sqlite", "postgres", "runtime-tokio", "tls-rustls"] }
 bitflags = "2"
 prost = "0.13"