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/style.css

@@ -470,6 +470,27 @@ button.primary:disabled { opacity: 0.5; cursor: not-allowed; }
 .relay-dot-small.green { background: var(--green); }
 .relay-dot-small.blue { background: #60a5fa; }
 
+/* ── Codec badges ── */
+.codec-badge {
+  display: inline-block;
+  font-size: 10px;
+  font-weight: 600;
+  padding: 1px 6px;
+  border-radius: 4px;
+  font-family: monospace;
+  margin: 0 2px;
+}
+
+.codec-badge.tx {
+  background: #22c55e30;
+  color: #4ade80;
+}
+
+.codec-badge.rx {
+  background: #3b82f630;
+  color: #60a5fa;
+}
+
 /* ── Controls ── */
 .controls {
   display: flex;