Fix embed player crash and prevent duplicate initialization

- Fix `TypeError: this.videoModal.resetStats is not a function` by implementing `resetStats` (no-op) in `EmbedVideoModal` and adding a safety guard in `Application.resetTorrentStats`.
- Prevent duplicate `NostrClient` initialization within the same frame by adding idempotent guards in `js/embed.js` and `js/nostr/client.js`.
- Add documentation (`docs/embed-notes.md`) on embed architecture and recommendations for multi-frame resource sharing (SharedWorker).
- Add regression test `tests/embed_playback_check.spec.mjs`.

js/embed.js

@@ -75,6 +75,10 @@ class EmbedVideoModal {
     return this.video;
   }
 
+  resetStats() {
+    // No-op for embed view
+  }
+
   updateStatus(message) {
     setStatus(message, "default");
   }
@@ -353,6 +357,11 @@ const setPointerState = ({ video, hint } = {}) => {
 };
 
 const startEmbed = async () => {
+  if (window.__bitvidEmbedStarted) {
+    return;
+  }
+  window.__bitvidEmbedStarted = true;
+
   if (!(videoEl instanceof HTMLVideoElement)) {
     setStatus("Embed video element is missing.", "error");
     return;