Per-website rule configuration

**What problem does this solve?**
Sometimes, I want to use different rules in different situations. Currently, the Harper extension only allows globally enabling or disabling rules, so I can't set specific rules for individual websites.

**Proposed Solution**
In addition to disabling a rule globally, I should be able to disable it "for this website only." This should appear in the settings somewhere as a list of sites with exceptions (maybe unified with the site's enable/disable switch).

**Examples**
I want Harper to check sentence capitalization on *most* websites, but on Discord, I don't particularly care. This leaves me with a choice: either I can disable the rule globally (bad for non-Discord), or I can keep it on and just ignore it (kind of annoying).

**Component**
- [ ] Core engine
- [x] Plugin/Extension
- [ ] Other: _____

The issue names only the Plugin/Extension component and does not identify files, tests, or an entry point. Start by locating the extension settings and existing global rule and site enable/disable configuration, then trace how website context is available. Done means users can disable an individual rule for selected websites while retaining the global setting, with the exception list exposed in settings.

Written by the indexing model from the issue text.