feat: bulk selection with checkbox column

[theAlexPatin]

Add useSelection hook with Set-based state. Add checkbox column in
FileList (select all + per-row) and checkbox overlay in FileGrid.
Bulk action bar in FileBrowser with Download (zip) and Delete actions.
Selection clears on navigation.

[theAlexPatin]

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• feat: add thumbnails in list view and larger grid thumbnails #29
• feat: add childCount to directory entries #28
• feat: add loading skeleton components #27
• feat: bulk selection with checkbox column #26 👈 (View in Graphite)
• feat: settings page tabs (Setup Guide / Access Tokens) #25
• feat: context menu via right-click and long-press #24
• feat: add admin page tabs (Storage / Users / System) #23
• feat: improve mobile grid density #22
• feat: add sort controls on list view column headers #21
• feat: mobile list row density — two-line rows #20
• feat: add toast notification system #19
• feat: add EmptyState component #18
• feat: add Badge component with semantic variants #17
• feat: add shared Button component with variants #16
• feat: add design tokens as CSS custom properties #15
• feat: touch target audit — enforce min 44px targets #14
• feat: add collapsible breadcrumb component for mobile #13
• feat: extract shared getFileIcon + file type icon colors #12
• main

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[theAlexPatin]

Superseded by #30 (combined UI redesign PR)