Save fetched lyrics to file alongside downloaded tracks

[TheZupZup]

Overview

Lyrics are already fetched and shown in the UI tab. The next step is saving them to disk so users have lyrics in their local music library without needing to fetch them again.

What's already done:

• LRCLIB fetching (synced + plain text) is implemented in src/Qrip.py
• The lyrics text is already available in memory after a successful fetch

What needs to be done

• After a track downloads successfully, check if lyrics were fetched for it
• If synced (timestamped) lyrics are available, save as <track-name>.lrc in the output folder
• If only plain text is available, save as <track-name>.txt instead
• If no lyrics were found, do nothing — don't create empty files
• Make sure the filename matches the downloaded audio file name

Testing steps

1. Download a track that has synced lyrics on LRCLIB → a .lrc file should appear next to the audio file
2. Download a track with only plain lyrics → a .txt file should appear
3. Download a track with no lyrics → no file created, no error shown

Goal

Lyrics are automatically saved alongside downloaded tracks in the correct format with no extra user action needed.

Related to: #73