Add "Count time on allowed sites" option

[MrDetox]

This PR adds an option so that time spent on exception URLs (sites with + in the block set) counts toward the set's time limit. Blocking and allowing of sites is unchanged; only what counts toward the limit changes.

Use case: The user blocks [youtube.com](http://youtube.com/) after 30 min but adds the exception +[youtube.com/watch](http://youtube.com/watch) so they can still watch specific YouTube videos. The idea is to allow only videos they deliberately open (e.g. from a Google search or a direct link), not browsing and discovery on YouTube itself. So they get about 30 minutes of YouTube in total, and after that only the main site is blocked; exception URLs like [youtube.com/watch](http://youtube.com/watch) stay allowed.

Current behaviour: While on YouTube, the 30-minute timer runs. The user can open and watch videos. While they watch videos on [youtube.com/watch](http://youtube.com/watch), the 30-minute timer does not run.

What the new option changes: When the new setting is enabled, the 30-minute timer also runs on exception URLs such as +[youtube.com/watch](http://youtube.com/watch). When the 30 minutes are used up, the main site is blocked; exception pages stay accessible and the timer is hidden on them.

Changes

• New option: "Count time spent on allowed sites (exceptions)" in Block Set options (Full Options only).
• When enabled, time on exception URLs counts toward the set's time limit.
• Exception URLs are still never blocked; when the limit is reached they remain open and the countdown is hidden on those pages.

Testing

• With the option enabled, time on exception URLs is counted.
• When the limit is reached, the main site is blocked and exception URLs stay open with no timer.
• The option is only visible in Full Options mode.