fix(calls): resolve EC mute hang, robust camera focus, PiP NaN guard (N122/N123/N126)

- N122: setMediaState resolves on EC's transport ACK instead of waiting for a
  DeviceMute state-echo that EC may elide or skip during teardown — which
  previously stranded the promise forever and silently skipped the initial
  deafen state + first StateUpdate on join. Dropped the single-slot
  mediaStatePromiseResolver; onMediaState remains the authoritative sync path.
- N123: focusCameraParticipant now waits for a spotlight videoTile to mount via
  a MutationObserver (with a 600ms hard-timeout fallback) instead of a fixed
  2-frame delay that EC's React commit can exceed on slower devices.
- N126: PiP position restored from localStorage is shape+finiteness validated,
  so corrupt data can't feed NaN into the position math (invalid 'NaNpx' CSS).

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

src/app/components/CallEmbedProvider.tsx

@@ -720,7 +720,25 @@ export function CallEmbedProvider({ children }: CallEmbedProviderProps) {
     if (pipMode) {
       if (!wasInPip) {
         const saved = localStorage.getItem('pip-position');
-        const savedPos = saved ? (JSON.parse(saved) as { left: number; top: number }) : null;
+        let savedPos: { left: number; top: number } | null = null;
+        if (saved) {
+          try {
+            const raw = JSON.parse(saved) as { left?: unknown; top?: unknown };
+            // Validate shape + finiteness: a corrupt value would otherwise feed
+            // NaN into Math.min and produce an invalid `NaNpx` CSS value.
+            if (
+              raw &&
+              typeof raw.left === 'number' &&
+              Number.isFinite(raw.left) &&
+              typeof raw.top === 'number' &&
+              Number.isFinite(raw.top)
+            ) {
+              savedPos = { left: raw.left, top: raw.top };
+            }
+          } catch {
+            savedPos = null;
+          }
+        }
         el.style.right = 'auto';
         el.style.bottom = 'auto';
         if (savedPos) {