Visitor Traffic Stats Pro

This add-on adds nothing to the dashboard by itself — the free plugin
already builds every widget. All this plugin does is:

1. Add a “License” box to the free plugin’s *Traffic Stats → Go Pro*
page (no separate settings screen to hunt for).
2. Validate the entered license key.
3. Tell the free plugin “Pro is unlocked” via the vtrs_is_pro filter
once a key validates.

With Pro active, the free plugin unlocks:
• Full country-level traffic tables (no top-5 cap)
• Real “Top Referring Countries” (visitors who arrived via an external
link — not a re-labelled copy of the general country table)
• Full traffic-source breakdown (direct / search / social / referral)
• CSV export of raw visit data
• 90-day trend history instead of 14