Protect private project activity

[bjarn]

Summary

• Apply item visibility checks to the activity page, recent activity, and activity widget.
• Exclude private comments from activity queries.
• Count only public comments in activity output.
• Add regression tests for private projects and private note counts.

Security impact

Activity feeds could disclose private project item metadata and private note counts.

Tests

• php artisan test --compact tests/Feature/Livewire/ActivityTest.php tests/Feature/Livewire/Welcome/RecentActivityTest.php tests/Feature/Widget/WidgetTest.php