feat(ui): add action bar

[MatteoGabriele]

🔗 Linked issue

resolves #1509

🧭 Context

Added a multi-select feature to the search page that allows users to select multiple packages and perform bulk actions on them. Currently supports comparing selected packages, with the possibility of adding more actions in the future.

📚 Description

• Selection UI: Added checkboxes to package cards that appear on hover. Selected cards are visually indicated with a border highlight and checkbox state.
• Persistent counter: New "View selected (X)" button in the toolbar shows active selections and navigates to a dedicated view for managing them.
• Floating action bar: When items are selected, a floating action bar appears at the bottom with the selection count, primary action (Compare), and clear button.
• Selection state management: Uses composable to maintain selections across view changes (card/table/selections view), allowing users to continue browsing while keeping their selections.
• Accessibility: Includes aria-live announcements for selection changes and keyboard shortcuts ("b" key) to focus the action bar.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
docs.npmx.dev	Ready	Preview, Comment	Feb 27, 2026 11:00am
npmx.dev	Ready	Preview, Comment	Feb 27, 2026 11:00am

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
npmx-lunaria	Ignored		Feb 27, 2026 11:00am

[github-actions]

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