1080p+ formats not available when using Brave cookies + NoneType bug

[Kazhwaan]

Description

• YTSage crashes with argument of type 'NoneType' is not iterable when yt-dlp returns no stdout (handled by adding defensive checks in ytsage_gui_analysis.py).
• When the user selects Brave as the cookie source, the program still uses the old config key browser_cookies instead of the current key cookie_browser. Result: only up to 720p formats are shown.

Steps to reproduce

1. Install YTSage and keep default config.
2. Open Settings → Cookies → Browser = Brave and enable “Remember”.
3. Analyze any YouTube video that has 1080p+ formats.
4. Only 720p and lower appear.

Expected behaviour

All available formats (including 1080p, 1440p, 4K) should be listed.

Actual behaviour

Only 720p and lower are shown.

Proposed fix

• Update ytsage_gui_main_source.py to write the selected browser to the key cookie_browser instead of browser_cookies.
• Ensure cookie_source is set to browser and cookie_remember is true so the setting persists.
• Add documentation in the settings dialog about Brave's cookie database lock issue (and suggest using Firefox or a manual cookies.txt file as fallback).
• Keep the defensive None checks that were already merged.

Additional notes

• If the user prefers Firefox instead of Brave, they can set cookie_browser: firefox in the config or through the UI.
• Providing a manual cookies.txt file works for any browser and avoids the lock problem.