feat(call): media health watchdog — warn user when no audio arrives

When a P2P direct call establishes successfully but the underlying
network path dies (phone switched from WiFi to LTE mid-call, or
cross-relay media forwarding isn't working), the call stays up
silently with recv_fr frozen at 0. No feedback to the user.

New watchdog in the Android recv task: tracks consecutive
heartbeat ticks (2s each) where recv_fr hasn't advanced. After 3
ticks (6s) with no new packets, emits:

- call-event { kind: "media-degraded" } — user-facing warning
  banner: "No audio — connection may be lost. Try hanging up and
  reconnecting, or switch to a different relay."
- call-debug media:no_recv_timeout for the debug log

If packets resume (recv_fr advances), clears the banner via:
- call-event { kind: "media-recovered" }

JS listener creates/removes a red-tinted banner dynamically at
the top of the call screen. Banner is also cleaned up on
showConnectScreen (call end).

This covers:
- Direct P2P that established on WiFi but died when the phone
  switched to LTE (stale NAT mapping, unreachable peer)
- Cross-relay calls where federation media isn't forwarding
  (relay not upgraded, not federated, etc.)
- Any other "connected but silent" scenario

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

desktop/src-tauri/src/engine.rs

@@ -653,6 +653,17 @@ impl CallEngine {
             let mut last_written: usize = 0;
             let mut decode_errs: u64 = 0;
             let mut first_packet_logged = false;
+            // Phase 5.6: media health watchdog — track consecutive
+            // heartbeat ticks where recv_fr hasn't advanced. If
+            // media doesn't arrive for 3 consecutive heartbeats
+            // (6s), emit a user-facing "media-degraded" call-event
+            // so the UI can show a warning like "No audio — try
+            // reconnecting?". Covers the case where P2P direct
+            // established but the underlying network path died
+            // (e.g., phone switched from WiFi to LTE mid-call).
+            let mut last_recv_fr_for_watchdog: u64 = 0;
+            let mut no_recv_ticks: u32 = 0;
+            let mut media_degraded_emitted = false;
 
             loop {
                 if !recv_r.load(Ordering::Relaxed) {
@@ -905,6 +916,59 @@ impl CallEngine {
                             "codec": format!("{:?}", current_codec),
                         }),
                     );
+
+                    // Media health watchdog: if recv_fr hasn't
+                    // advanced in 3 consecutive heartbeats (6s) and
+                    // we've been "connected" for at least 4s (give
+                    // the first few frames time to arrive), emit a
+                    // user-facing "media-degraded" event so the UI
+                    // can show "No audio — connection may be lost".
+                    if fr == last_recv_fr_for_watchdog {
+                        no_recv_ticks += 1;
+                    } else {
+                        no_recv_ticks = 0;
+                        if media_degraded_emitted {
+                            // Was degraded but recovered — clear
+                            // the banner.
+                            media_degraded_emitted = false;
+                            let _ = recv_app.emit(
+                                "call-event",
+                                serde_json::json!({
+                                    "kind": "media-recovered",
+                                }),
+                            );
+                            crate::emit_call_debug(
+                                &recv_app,
+                                "media:recovered",
+                                serde_json::json!({}),
+                            );
+                        }
+                    }
+                    last_recv_fr_for_watchdog = fr;
+
+                    if no_recv_ticks >= 3 && !media_degraded_emitted {
+                        media_degraded_emitted = true;
+                        tracing::warn!(
+                            recv_fr = fr,
+                            no_recv_ticks,
+                            "media watchdog: no inbound packets for 6s"
+                        );
+                        let _ = recv_app.emit(
+                            "call-event",
+                            serde_json::json!({
+                                "kind": "media-degraded",
+                            }),
+                        );
+                        crate::emit_call_debug(
+                            &recv_app,
+                            "media:no_recv_timeout",
+                            serde_json::json!({
+                                "recv_fr": fr,
+                                "no_recv_ticks": no_recv_ticks,
+                            }),
+                        );
+                    }
+
                     heartbeat = std::time::Instant::now();
                 }
             }

desktop/src/main.ts

@@ -887,6 +887,9 @@ function showConnectScreen() {
   connectBtn.textContent = "Connect";
   levelBar.style.width = "0%";
   directCallPeer = null;
+  // Clear the media-degraded banner if present
+  const banner = document.getElementById("media-degraded-banner");
+  if (banner) banner.remove();
   if (statusInterval) { clearInterval(statusInterval); statusInterval = null; }
 }
 
@@ -1088,6 +1091,42 @@ listen("call-event", (event: any) => {
   const { kind } = event.payload;
   if (kind === "room-update") pollStatus();
   if (kind === "disconnected" && !userDisconnected) pollStatus();
+
+  // Phase 5.6: media health watchdog — show/clear a warning
+  // banner when the media path dies (e.g., P2P direct
+  // established but the network path changed, or cross-relay
+  // media forwarding isn't working).
+  if (kind === "media-degraded") {
+    // Show a warning banner on the call screen. Don't auto-
+    // disconnect — the user might be on a briefly-unstable
+    // network and recovery is possible (the engine tracks
+    // "media-recovered" and clears the banner if packets
+    // resume).
+    let banner = document.getElementById("media-degraded-banner");
+    if (!banner) {
+      banner = document.createElement("div");
+      banner.id = "media-degraded-banner";
+      banner.style.cssText =
+        "background:rgba(239,68,68,0.15);color:var(--red);padding:8px 12px;" +
+        "border-radius:8px;text-align:center;font-size:13px;margin:8px 0;";
+      banner.innerHTML =
+        '⚠ No audio — connection may be lost.<br>' +
+        '<small style="color:var(--text-dim)">Try hanging up and reconnecting, or switch to a different relay.</small>';
+      // Insert at the top of the call screen, below the header
+      const participants = document.getElementById("participants");
+      const directView = document.getElementById("direct-call-view");
+      const insertBefore = (directView && !directView.classList.contains("hidden"))
+        ? directView
+        : participants;
+      if (insertBefore?.parentNode) {
+        insertBefore.parentNode.insertBefore(banner, insertBefore);
+      }
+    }
+  }
+  if (kind === "media-recovered") {
+    const banner = document.getElementById("media-degraded-banner");
+    if (banner) banner.remove();
+  }
 });
 
 // ── Settings ──