Add site preferences checklist to manage bundled default sites

**What problem does this solve?**
Harper enables linting on 53 bundled sites by default. Users cannot see which sites have an explicit setting or turn them off in bulk.

**Proposed Solution**
Add a "Site Preferences" section to the options page: a searchable checklist of the bundled defaults with bulk enable/disable and per-site toggles, plus a list of custom sites with explicit settings that can be reset.

**Examples**
User only wants Harper on GitHub: open options, disable all default sites, then check GitHub.

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

**Additional Context**
Domain preference keys are normalized to 'www.-stripped' hostnames so legacy 'www.' keys cannot shadow newer settings.

Start at the extension's options page and trace how bundled default-site preferences and custom site settings are represented. Implement the searchable Site Preferences checklist with bulk enable/disable, per-site toggles, and reset controls for custom sites. Done means users can disable all defaults, enable a selected site such as GitHub, and reset explicit custom settings.

Written by the indexing model from the issue text.