Call the sorting card Categories

"What sorts your mail" named two of the card's three controls. With
the tabs switch on it as well, the card is about categories as a
whole: whether the inbox shows them, which one a message lands in,
and who decides.

Refs #29

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/features/mail.md

@@ -82,7 +82,7 @@ re-categorisation never needs a resync.
 ### An inbox without tabs
 
 If you would rather have one list, switch **Inbox tabs** to **One list** under **Settings → General
-→ What sorts your mail**. The inbox then shows everything in it, newest first, and the tabs are
+→ Categories**. The inbox then shows everything in it, newest first, and the tabs are
 gone along with what belongs to them: the "new" pills, dragging a conversation onto a tab, and the
 panel that explains why a message was filed where it was.
 
@@ -93,7 +93,7 @@ choosing **Rules** stops it.
 
 ### Choosing what sorts your mail
 
-Both of those defaults are yours to change, under **Settings → General → What sorts your mail**.
+Both of those defaults are yours to change, under **Settings → General → Categories**.
 
 **Sorted by** picks what decides. **Rules** reads headers — a mailing-list header, an unsubscribe
 link, a sender you have written to before — and answers the same way every time for the same