feat(yts): Add youtube cookies support

[Dolly132]

Solves #68

[Rushaway]

Hey @Dolly132, thanks for the PR! Small, focused, and does exactly one thing — that's the right approach. Here are my thoughts:

✅ What's good

• Great scope: 3 files, ~25 lines, single clear objective
• Consistent propagation of the cookies param throughout the whole call chain (get_url_youtube_info, get_first_valid_entry, and all call sites in Commands.py)
• The os.path.isfile(cookies) guard is a nice touch — avoids cryptic yt-dlp errors if the file doesn't exist
• Opt-in by default ("" default value), so it's fully backward-compatible

⚠️ A few things worth addressing

1. Silent failure when cookies file is missing — if cookies is set in the config but the file doesn't exist, it gets reset to "" with no feedback. A warning log would really help with debugging:

   if cookies and not os.path.isfile(cookies):
       self.logger.warning(f"Cookies file not found: {cookies}, ignoring")
       cookies = ""

2. The default path in config.json — having "config/cookies.txt" as the example value is a bit dangerous: someone might copy the config as-is, the file won't exist, and cookies will be silently ignored (see point above). Maybe use an empty string "" as the default instead, or add a comment.

3. Check if the file is not empty - having "config/cookies.txt" is correct check, but the fail be empty and can lead to a silent failure. A warning log would really help with debugging

4. No documentation — the cookies file format (Netscape HTTP Cookie File) is not obvious for end users. A short note in the README about how to generate it (e.g. via a browser extension like "Get cookies.txt") would go a long way.

Overall this is solid and mergeable. The silent failure on missing file is the one thing I'd fix before merging — the rest are nice-to-haves. Good work!