fix(nav): Timeline/Grid active state via aria-selected (htmx-robust)
Root cause: body-attribute CSS (body[data-feed-view]) does NOT react reliably
after an htmx OOB swap; element attributes do. Therefore the active Timeline/Grid
state is now via [aria-selected="true"] (set by syncAria, which fires on
htmx:afterSettle) instead of body[data-feed-view]. syncAria sets aria-selected
only on feed pages → on calendar/downloads/post both are grey. Template also
sets the initial aria-selected neutral on non-feed pages.
Co-Authored-By: Claude <noreply@…>