feat: toggleable columns + version column on gardens index
Users can show/hide columns via a Columns dropdown above the gardens
table. Selection is persisted in the URL as ?cols=... so views are
shareable and survive reload. The existing :version field is exposed as
the first togglable column; Name is locked on so the table can't be
emptied.
Column visibility and Flop sort/page state coexist because cols= is
baked into the path passed to the table and pagination components;
Flop.Phoenix.build_path preserves existing query params on the base.
feat(sow-177): move Columns toggle to header as icon button
Relocate the Columns toggle into the page header's :actions slot as an
icon-only button (hero-view-columns) with aria-label/title. Reclaims the
vertical space previously taken by a dedicated row above the table.
sow-177
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>