feat(ui): sortable Name & Start time columns, persist user sorting across navigation

[xxxliqu1dxxx]

Three related improvements to table sorting in the htmlui:

1. feat(ui): make the Name column sortable in the directory listing

   • Adds accessorFn + a custom sortingFn to the Name column in
     DirectoryItems.jsx.
   • File-manager style: directories before files (both directions),
     case-insensitive
     compare with exact-compare fallback; sortDescFirst so first click sorts
     descending, consistent with the other columns.

2. feat(ui): persist user-applied table sorting across navigation

   • New TableSortContext keyed by a tableKey passed to KopiaTable; provider
     wraps
     the routes in App.jsx. Sort state now survives folder drill-down and
     cross-page navigation (directory, snapshots, snapshot-history, policies,
     tasks).
   • The context setter resolves TanStack v8's updater-function form of
     onSortingChange (typeof update === "function" ? update(current) : update).

3. feat(ui): make the Start time column sortable in the snapshot history

   • Adds accessorFn: (x) => x.startTime to the Start time column in
     SnapshotHistory.jsx. First click is ascending (v8 auto-detects 'asc' for
     strings), a visible flip from the default newest-first order.

Known nuance: in descending Name sort, files appear before directories because
TanStack v8's getSortedRowModel globally inverts the comparator and sortingFn
receives no direction. Ascending (the default after first click on any column
with sortDescFirst) shows dirs-first. Happy to iterate on this if maintainers
feel strongly about it.

All commits are GPG-signed.