feat(webui): virtualize data tables to handle thousands of rows - #360

List views rendered every row into the DOM, so a shared-files list with
thousands of entries put thousands of <tr> (with selects/progress bars)
through Preact's diff on each SSE update, locking the main thread.

Add a shared, column-config-driven VirtualTable (js/table.js) that windows
the visible rows plus overscan with spacer rows, using table-layout:fixed
for stable column widths (with a computed min-width so narrow screens scroll
horizontally instead of collapsing), a fixed row height, sticky thead, and
keyed rows. It also owns sortable headers, deduplicating the Th/cmp/sortVal
helpers that were copy-pasted across four views.

Migrate shared, downloads, search, servers and peers to it. Make the peers
table sortable (default: busiest first) and add a text filter to the
downloads, shared and peers tables, mirroring the search filter via a shared
textMatcher helper.

Fix multi-select so it stays consistent with the active filter: select-all
now acts only on the filtered rows, the selection is pruned to the visible
set when the filter changes (never keeps hidden rows selected), and a
"Selected N" count is shown next to the bulk actions.