fix(ui): name the section after its button, drop the folder slash, colour the widget

**"Zone sensible" said nothing.** The heading is now the name of the action in
it — "Quitter le groupe", or "Supprimer le groupe" for the owner, who sees a
different button. Worth stating because it is not quite what was asked for: a
fixed "Quitter le groupe" would have sat above a delete button for whoever owns
the group. The red goes with it; only the button is red, which is where the
warning belongs. `members.danger_title` is gone from all ten locales rather than
left behind unread.

**A folder name no longer ends in a slash.** The folder icon in the cell beside
it already says what it is.

**The transfers widget turns green while transfers run.** The badge counts them,
but a count has to be read; colour is what carries from across the room, which
is the point of a widget in the nav bar rather than on the page. Derived from
the live list on every render, so there is no state that can forget to clear
when the last transfer ends.

The class is set in `app.js` and coloured in `style.css` — either alone does
nothing and neither fails loudly, so there is a test for each half.

848 tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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