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)

protocol/lnex.proto

@@ -64,6 +64,11 @@ message ChannelCreate {
 
 message ChannelDelete { string channel_id = 1; }
 
+message ChannelEdit {
+  string channel_id   = 1;
+  string channel_name = 2;
+}
+
 message ChannelList { repeated ChannelListItem channels = 1; }
 
 message ChannelListItem {
@@ -359,3 +364,27 @@ message SimpleResponse {
   optional string user_id  = 2;
   optional string role_id  = 3;
 }
+
+// ─── E2EE for Direct Messages ────────────────────────────────────────────────
+
+message E2eeDmKeyExchange {
+  string dm_id           = 1;
+  bytes  e2ee_public_key = 2;
+}
+
+message E2eeDmKeyExchangeAck {
+  string dm_id = 1;
+}
+
+message E2eeDmMessage {
+  string dm_id     = 1;
+  string sender_id = 2;
+  bytes  ciphertext = 3;
+  int64  timestamp  = 4;
+}
+
+message E2eeDmHistory {
+  string          dm_id         = 1;
+  repeated E2eeDmMessage messages = 2;
+  optional int64  limit         = 3;
+}