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/components/voice/connection-card.tsx

@@ -119,10 +119,19 @@ export function VoiceConnectionCard({
           </SelectTrigger>
           <SelectContent>
             {voiceChannels.map((c) => (
-              <SelectItem key={c.id} value={c.id}>
-                {c.name}
+              <SelectItem
+                key={c.id}
+                value={c.id}
+                disabled={c.joinable === false}
+              >
+                {c.joinable === false ? `${c.name} (no akses)` : c.name}
               </SelectItem>
             ))}
+            {voiceChannels.length === 0 && (
+              <div className="px-3 py-2 text-xs text-text-secondary/60">
+                Tidak ada voice channel
+              </div>
+            )}
           </SelectContent>
         </Select>
       </div>