feat: allow users to choose card activity order

[bionicman]

Description

Adds a compact toggle to the card Activity header so users can switch the
complete timeline between oldest-first and newest-first order. Oldest-first
remains the default, and the choice is stored in the browser for the first
iteration agreed in #596.

The comment editor follows the newest edge of the feed. The API and repository
support both directions with a timestamp plus public-ID cursor, preserving a
stable order when activities share a timestamp. Newly submitted comments also
remain visible when older activity is behind the pagination boundary.

Screenshots

Newest-first:

Oldest-first:

Mobile

Type of change

• Bug fix
• Feature (requires an approved issue — see below)
• Refactor / chore
• Documentation

Checklist

• I have linked the related issue below
• My code follows the existing style and conventions
• I have tested my changes locally
• I have included screenshots for any UI changes

Linked issue

Closes #596

Feature PRs without a linked, approved issue will be closed without review.
Open or comment on a feature request first and wait for maintainer approval before building.