Add Startpage support; refactor search extraction

[yoasif]

• Implemented Startpage detection and sidebar injection
• Added automatic theme detection for Startpage "Air" and "Night" modes
• Unified search term extraction to support both 'q' and 'query' params
• Improved config loading to prevent data loss during updates

Closes #23

[Fivefold]

Looks great! I really appreciate you taking the time to implement startpage support.

Also appreciate the improvements/fixes for the config loading and search term extraction. I can't test at the moment, so I just have a few questions before merging:

1. Since I don't personally use startpage (but I assume you do), would you be willing to handle startpage-related fixes in the future, e.g. if startpage changes their html?
2. Can you make two screenshots for light and dark modes on startpage, showing the injection box?
3. Did you check the various permutations in theme settings (e.g. setting startpage's theme to light but injector theme to dark, setting injector theme to auto detect and startpage to system theme, changing the system theme to light and to dark)?
   Here is a table for the various permutations, with startpage theme settings horizontally and injector theme settings vertically:

	System (Light)	System (Dark)	Light	Dark
Auto
Light
Dark