fix(player): tell the user when the server cannot read the media file

A 404 on the media stream means the server resolved the item but could not
open the file behind it — moved, deleted, or on storage that went away.
Jellyfin maps the resulting FileNotFoundException to 404, and PlaybackInfo
never stats the file, so negotiation succeeds and only the stream request
fails. Playback then died with a snackbar reading "Failed to open
[REDACTED_URL]" before popping the route, which tells the user nothing and
leaves nothing useful in a bug report.

Generalize the HTTP-500 log probe into PlayerError.httpStatusFromLog and
latch every status in fatalPlaybackHttpStatuses. Each latches on its own so
the 503 that stream-lavf-o deliberately retries cannot mask the fatal status
behind it. A 404 now raises a dedicated modal naming the cause and the fix.

On Android a 404 previously failed the "Response code: 500" string test and
fell through to the ExoPlayer→MPV fallback, showing "switching to compatible
player" before failing again on the same request. Read the real status off
HttpDataSource.InvalidResponseCodeException instead and skip the fallback:
an HTTP status is not a codec problem.

lib/i18n/zh.i18n.json

@@ -694,6 +694,8 @@
     "switchingToCompatiblePlayer": "正在切换到兼容的播放器…",
     "serverLimitTitle": "播放失败",
     "serverLimitBody": "服务器错误（HTTP 500）。此次会话可能因带宽或转码限制而被拒绝。请联系服务器所有者调整限制。",
+    "mediaUnreadableTitle": "",
+    "mediaUnreadableBody": "",
     "logsUploaded": "日志已上传",
     "logsUploadFailed": "上传日志失败",
     "logId": "日志 ID"