fix(calls,matrix): address review findings from agent code review

- CallEmbed watchdog now SELF-HEALS: a genuine ready/joined signal arriving
  after the 25s timeout clears the error and notifies subscribers with
  undefined, so a slow-but-successful EC load no longer strands the user on
  the recovery screen over a live call. Listener dispatch wrapped in try/catch.
- ringtones: synth notes route through a per-session master gain; stop() ramps
  it to 0 so the ring is silenced instantly on answer instead of letting the
  last scheduled phrase ring out over call audio.
- IncomingCallBanner: ping fires exactly once per incoming call (guarded by
  refEventId) instead of re-pinging when ringtone settings change mid-banner.
- focusCameraParticipant: try multiple tile selectors (EC labels vary by
  version), defer the tile click past EC's async spotlight layout switch
  (rAF x2), and dev-warn when no tile matches so testers get signal.
- uploadContent: a cancelled upload (mx.cancelUpload -> AbortError) is no
  longer treated as retryable — previously the retry loop could resurrect an
  upload the user just cancelled. Also retry on 408.
- addRoomIdToMDirect/removeRoomIdFromMDirect: guard against a corrupt m.direct
  whose values aren't arrays.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/utils/ringtones.ts

@@ -78,7 +78,7 @@ const PHRASES: Record<
   },
 };
 
-const playPhrase = (style: SynthStyle, volume: number): void => {
+const playPhrase = (style: SynthStyle, volume: number, destination: AudioNode): void => {
   const ctx = getCtx();
   if (!ctx) return;
   const { type, gain: peak, notes } = PHRASES[style];
@@ -96,7 +96,7 @@ const playPhrase = (style: SynthStyle, volume: number): void => {
     gain.gain.linearRampToValueAtTime(scaledPeak, start + 0.015);
     gain.gain.exponentialRampToValueAtTime(0.0001, start + dur);
     osc.connect(gain);
-    gain.connect(ctx.destination);
+    gain.connect(destination);
     osc.start(start);
     osc.stop(start + dur + 0.02);
   });
@@ -121,11 +121,41 @@ const startClassic = (volume: number, loop: boolean): (() => void) => {
 };
 
 const startSynth = (style: SynthStyle, volume: number, loop: boolean): (() => void) => {
-  playPhrase(style, volume);
-  if (!loop) return () => undefined;
-  const period = PHRASES[style].period * 1000;
-  const id = window.setInterval(() => playPhrase(style, volume), period);
-  return () => window.clearInterval(id);
+  const ctx = getCtx();
+  if (!ctx) return () => undefined;
+  // All notes route through a per-session master gain so stop() can silence
+  // everything instantly — including notes already scheduled slightly in the
+  // future — instead of letting the last phrase ring out after the user answers.
+  const master = ctx.createGain();
+  master.gain.value = 1;
+  master.connect(ctx.destination);
+
+  playPhrase(style, volume, master);
+  const id = loop
+    ? window.setInterval(() => playPhrase(style, volume, master), PHRASES[style].period * 1000)
+    : 0;
+
+  let stopped = false;
+  return () => {
+    if (stopped) return;
+    stopped = true;
+    if (id) window.clearInterval(id);
+    try {
+      const now = ctx.currentTime;
+      master.gain.cancelScheduledValues(now);
+      master.gain.setValueAtTime(master.gain.value, now);
+      master.gain.linearRampToValueAtTime(0, now + 0.03);
+    } catch {
+      /* context may be closed */
+    }
+    window.setTimeout(() => {
+      try {
+        master.disconnect();
+      } catch {
+        /* already disconnected */
+      }
+    }, 100);
+  };
 };
 
 /**