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/proto/packet/id.rs

@@ -16,6 +16,7 @@ pub enum PacketId {
     ChannelCreate = 0x0033,
     ChannelDelete = 0x0034,
     ChannelList = 0x0035,
+    ChannelEdit = 0x0036,
     UserJoin = 0x0040,
     UserLeave = 0x0041,
     PermissionCheck = 0x0050,
@@ -24,6 +25,10 @@ pub enum PacketId {
     DmMessage = 0x0061,
     DmHistory = 0x0062,
     DmReadAck = 0x0063,
+    E2eeDmKeyExchange = 0x0071,
+    E2eeDmKeyExchangeAck = 0x0072,
+    E2eeDmMessage = 0x0073,
+    E2eeDmHistory = 0x0074,
     ReadReceipt = 0x0064,
     ReadReceiptBroadcast = 0x0065,
     MessageReactionAdd = 0x0066,
@@ -84,6 +89,7 @@ impl PacketId {
             0x0033 => Some(Self::ChannelCreate),
             0x0034 => Some(Self::ChannelDelete),
             0x0035 => Some(Self::ChannelList),
+            0x0036 => Some(Self::ChannelEdit),
             0x0040 => Some(Self::UserJoin),
             0x0041 => Some(Self::UserLeave),
             0x0050 => Some(Self::PermissionCheck),
@@ -99,6 +105,10 @@ impl PacketId {
             0x0068 => Some(Self::MessageEdit),
             0x0069 => Some(Self::MessageDelete),
             0x0070 => Some(Self::TypingStart),
+            0x0071 => Some(Self::E2eeDmKeyExchange),
+            0x0072 => Some(Self::E2eeDmKeyExchangeAck),
+            0x0073 => Some(Self::E2eeDmMessage),
+            0x0074 => Some(Self::E2eeDmHistory),
             0x0100 => Some(Self::GuildCreate),
             0x0101 => Some(Self::GuildDelete),
             0x0102 => Some(Self::GuildList),