docs: media player subsystem architecture and conventions

Document the media server process, /media/ port map, library layout on
the ./media bind mount, Hope's media MCP toolset, the music/mic
mutual-exclusion rule, and yt-dlp download/proxy conventions.

web/app.js

@@ -1263,7 +1263,13 @@ function clearMediaAudio() {
 
 function updateMediaNowCard(track) {
     mediaTitleEl.textContent = track.title || track.id;
-    mediaChannelEl.textContent = track.channel || (track.kind === "stream" ? "Live stream" : "");
+    if (track.status === "failed") {
+        mediaChannelEl.textContent = "Download failed — pick another result or try again.";
+    } else if (track.status === "downloading") {
+        mediaChannelEl.textContent = "Preparing…";
+    } else {
+        mediaChannelEl.textContent = track.channel || (track.kind === "stream" ? "Live stream" : "");
+    }
     if (track.art_url) {
         mediaArt.src = track.art_url;
         mediaArt.hidden = false;
@@ -1403,7 +1409,9 @@ mediaAudio.addEventListener("timeupdate", () => {
 });
 mediaAudio.addEventListener("durationchange", updateSeekUI);
 mediaAudio.addEventListener("error", () => {
-    if (mediaCurrentTrackId) {
+    // An empty src (track still downloading) also fires this — only surface
+    // real playback failures.
+    if (mediaCurrentTrackId && mediaAudio.getAttribute("src")) {
         mediaChannelEl.textContent = "Playback error — the track may still be preparing.";
     }
 });