Add titled playlist chooser with active selection in BGM tray

[h4rm0n1c]

Motivation

• Provide a YAD picker that shows playlist/video titles (instead of raw URLs) so users can pick a playlist by name.
• Inform the user which playlist is currently loaded by pre-selecting the active source when the chooser opens.

Description

• Added CONFIG_URL_FILE handling and a read_current_url() helper to read the currently active URL from streamlink_bgm_url.
• Added resolve_source_title() to resolve playlist/video titles using yt-dlp (with an 8s timeout when timeout is available) and fall back to the URL if resolution fails.
• Reworked choose_bgm_url() to build a temporary list, present a yad --radiolist with columns Current, Playlist (title), and URL, pre-select the active URL, and call streamlink_3.sh seturl with the selected URL.
• Exported the new helper functions and updated documentation in streamlinkbgm/README.md and docs/streamlinkbgm.md to document the new chooser behavior.

Testing

• Ran bash -n streamlinkbgm/bgm_tray.sh and the syntax check completed successfully.
• Ran bash -n streamlinkbgm/streamlink_3.sh and the syntax check completed successfully.

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Codex Task