fix(player): surface a persistent HTTP 503 at open instead of retrying forever

ffmpeg's reconnect loop deliberately retries 503 without bound (#1520), so a
server that keeps refusing the stream at open time left a silent black screen:
ExoPlayer fell back to MPV, MPV reconnected forever, and no error ever reached
the screen. A new open-phase watchdog arms on the first 503 seen before any
frame renders and, after 20s without one, synthesizes a server-http-503 error
that shows an actionable dialog. Mid-stream 503s and live TV keep their
existing ride-out paths.

close #1830

lib/i18n/en.i18n.json

@@ -744,6 +744,8 @@
     "serverLimitBody": "Server error (HTTP 500). A bandwidth/transcoding limit likely rejected this session. Ask the owner to adjust it.",
     "mediaUnreadableTitle": "File unavailable",
     "mediaUnreadableBody": "The server found this item but could not read its file (HTTP 404). The file was probably moved, deleted, or its storage is offline. Ask the server owner to check the file and rescan the library.",
+    "serverBusyTitle": "Stream unavailable",
+    "serverBusyBody": "The server kept refusing to stream this file (HTTP 503). It may be restarting, busy, or the file's storage may be offline. Try again in a moment — if it keeps happening, ask the server owner to check the server and the file's storage.",
     "logsUploaded": "Logs uploaded",
     "logsUploadFailed": "Failed to upload logs",
     "logId": "Log ID"