Reduced permissions (privacy improvement)

Earlier versions of TextGrab took a broad "Access your data for all websites" permission because the content scripts ran on every page you visited. This update fixes that.

What changed:
- The extension now uses Firefox's activeTab permission instead of a permanent all-sites permission.
- Page access is granted temporarily, one tab at a time, and only when you deliberately use TextGrab (click the toolbar button or a right-click menu item).
- No more persistent access to every website you browse; data is only read from the current page when you invoke the extension.

What's the same:
- Extract clean article text + convert to markdown
- One-click copy to clipboard
- Context menu options
- No data collection — everything runs locally

Why: This narrows the permission footprint so TextGrab only ever reads the page you're actively working on, when you choose to — nothing always-on in the background.