Proxy YouTube playback through server to fix intermittent 403s

Direct googlevideo URLs are bound to the extractor's IP and expire, so the
browser fetching them from a different IP got intermittent 403s on playback.
Route playback through a same-origin /api/play proxy that fetches the stream
server-side (matching the extractor IP) with yt-dlp's own http_headers and
forwards Range headers for seeking; fall back to piping yt-dlp for SABR/itag-18
formats a plain GET can't fetch. Prefer adaptive streams over progressive in
the playback fallback order.

frontend/app.js

@@ -1441,9 +1441,11 @@ const Player = {
     }
   },
 
-  // Ordered list of qualities to try if playback errors out. Progressive
-  // (single-file, has audio) goes near the front because it's the most reliable;
-  // then we walk from the lowest resolution up.
+  // Ordered list of qualities to try if playback errors out. Adaptive
+  // (video-only + separate audio via /api/play) goes near the front: those
+  // streams proxy reliably, whereas progressive single-file formats (itag 18
+  // and friends) are increasingly SABR-gated and 403 even server-side, so they
+  // now go LAST. We then walk from the lowest resolution up.
   fallbackQueue: [],
   fbIndex: 0,
   buildFallbackQueue(chosen) {
@@ -1452,8 +1454,8 @@ const Player = {
     const queue = [];
     const push = (x) => { if (x && !seen.has(x.label)) { seen.add(x.label); queue.push(x); } };
     push(chosen);
-    qs.filter((q) => q.hasAudio).forEach(push); // progressive = sturdiest
-    qs.forEach(push);                            // then everything, low → high
+    qs.filter((q) => !q.hasAudio).forEach(push); // adaptive = sturdiest (proxied)
+    qs.forEach(push);                            // then everything, incl. progressive
     this.fallbackQueue = queue;
     this.fbIndex = 0;
   },