Changelog timeline redesigned; per-entry tags link back to filtered timeline

Changelog timeline redesign

Changelog pages now render as a compact, date-grouped timeline of entry cards, replacing the previous layout that stacked full-height entries inline.

A search bar at the top lets readers find entries across the entire changelog by keyword. The query syncs to the ?q= URL parameter, so filtered views are shareable.

Per-entry changelog tags

Changelog entries with multiple releases under separate headings can tag each heading independently using the <ChangelogTags> component:

## New dashboard analytics
<ChangelogTags>analytics, dashboard</ChangelogTags>

Tags on individual entry pages are clickable and link back to the timeline with that tag pre-selected as a filter.