fix(voice): media queue field mismatch, voice joinable filter, connect error toast

Audit voice (kirim/terima/music/screenshare) menemukan 3 masalah:
1. media:queue SILENT no-op — backend publish {source,mode} tapi gateway
   handler baca payload.url → selalu 'received without a URL'. Backend
   sekarang kirim {url,mode}, gateway terima url ATAU source (robust).
2. Voice connect gagal diam-diam saat user pilih channel tanpa permission
   (joinable=false, contoh Music 32/64/128/256k). Backend+gateway sekarang
   expose joinable; FE disable channel 'no akses' + empty state.
3. FE tidak kasih feedback saat connect gagal — tambah toast.error dengan
   pesan dari backend.

Verified live: @discordjs/voice connect ke Lofi Radio joinable sukses
(VOICE READY, DAVE session OK) — pipeline voice sebenarnya sehat, masalah
utama UX. media:queue fix akan di-verify setelah deploy.

services/frontend/src/app/(dashboard)/voice/page.tsx

@@ -18,6 +18,7 @@ import {
   useVoiceStatus,
 } from "@/hooks";
 import { useWebSocket } from "@/lib/ws/context";
+import { toast } from "sonner";
 
 type VoiceTab = "connection" | "activity";
 
@@ -104,12 +105,22 @@ export default function VoicePage() {
         selectedChannel={selectedChannel}
         onGuildChange={handleGuildChange}
         onChannelChange={(v) => setSelectedChannel(v ?? "")}
-        onConnect={() =>
-          connectMut.mutate({
-            guildId: selectedGuild,
-            channelId: selectedChannel,
-          })
-        }
+        onConnect={() => {
+          void connectMut
+            .mutateAsync({
+              guildId: selectedGuild,
+              channelId: selectedChannel,
+            })
+            .catch((err: unknown) => {
+              const msg =
+                err instanceof Error
+                  ? err.message
+                  : "Gagal connect ke voice channel";
+              toast.error("Voice connect gagal", {
+                description: msg,
+              });
+            });
+        }}
         onDisconnect={() => {
           if (micActive) {
             setMicActive(false);