New: per-site toggle
- The popup now has a "Disable on this site" checkbox. Useful when a site already uses a monospace font you'd rather keep, or when overriding breaks the layout. The setting is per-origin and persists across tabs.

Improvements:
- Toggling the per-site setting now auto-reloads the current tab (no more manual reload).
- Removed the "Reload tabs to apply changes" hint — the popup is now self-explanatory.

Under the hood:
- Added activeTab permission, used solely to read the current tab's origin when you open the popup. No browsing data leaves the extension.
- Declared data_collection_permissions: ["none"] to comply with Mozilla's new manifest requirement.