Report client fingerprint on playback failure

showStreamError + playerFail now POST the failing browser's UA,
hls.js presence, MSE support and native-HLS capability to the
events table — a broken mobile browser becomes diagnosable from
the server side instead of by guessing.

static/app.js

@@ -70,6 +70,15 @@ function toast(msg, action) {
 function showStreamError(msg, retry) {
   // visible overlay + Retry — never leave a silent spinner (this is what
   // an unplayable browser used to hit: 'HLS not supported' under the overlay)
+  // + report the client's capabilities so a failing browser is diagnosable
+  // from server.log instead of by guesswork
+  try {
+    postEvent({type: "client-fail", msg,
+      ua: navigator.userAgent,
+      hls_lib: typeof window.Hls,
+      mse: (window.Hls && Hls.isSupported) ? Hls.isSupported() : null,
+      native_hls: document.createElement("video").canPlayType("application/vnd.apple.mpegurl") || "none"});
+  } catch {} 
   playerOverlay("Stream error: " + msg, retry || (() => { if (currentEp) play(currentEp.ep_id, currentEp.ep_no); }), "Retry");
 }
 
@@ -752,7 +761,7 @@ function playerFail(msg) {
   // the last 10s save when they leave
   savePos();
   const { ep_id, ep_no } = currentEp;
-  postEvent({type: "error", detail: msg, ep_id, ep: ep_no, lang});
+  postEvent({type: "error", detail: msg, ep_id, ep: ep_no, lang, ua: navigator.userAgent});
   playerOverlay("Playback failed: " + msg, () => play(ep_id, ep_no));
 }