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/uz.i18n.json

@@ -744,6 +744,8 @@
     "serverLimitBody": "Server xatoligi (HTTP 500). Cheklov ushbu seansni rad etdi.",
     "mediaUnreadableTitle": "Fayl mavjud emas",
     "mediaUnreadableBody": "Server bu elementni topdi, lekin uning faylini oʻqiy olmadi (HTTP 404). Fayl koʻchirilgan, oʻchirilgan yoki uning saqlash joyi oflayn boʻlishi mumkin. Server egasidan faylni tekshirishni va kutubxonani qayta skanerlashni soʻrang.",
+    "serverBusyTitle": "",
+    "serverBusyBody": "",
     "logsUploaded": "Jurnallar yuklandi",
     "logsUploadFailed": "Jurnallarni yuklab boʻlmadi",
     "logId": "Jurnal ID-si"