feat: TX/RX codec badges on desktop call screen

Desktop now shows codec badges like Android:
- Green TX badge: e.g. "Opus64k"
- Blue RX badge: e.g. "Opus24k"
Displayed in the stats line below the call controls.

Engine tracks tx_codec (set on encoder init) and rx_codec (updated
from incoming packet headers). Passed through EngineStatus → CallStatus
→ frontend.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/src/main.ts

@@ -586,7 +586,10 @@ async function pollStatus() {
       });
     }
 
-    statsDiv.textContent = `TX: ${st.encode_fps} | RX: ${st.recv_fps}`;
+    // Stats line with codec badges
+    const txBadge = (st as any).tx_codec ? `<span class="codec-badge tx">${escapeHtml((st as any).tx_codec)}</span>` : "";
+    const rxBadge = (st as any).rx_codec ? `<span class="codec-badge rx">${escapeHtml((st as any).rx_codec)}</span>` : "";
+    statsDiv.innerHTML = `${txBadge} ${rxBadge} TX: ${st.encode_fps} | RX: ${st.recv_fps}`;
   } catch {}
 }