feat: mark the boundary between forums and topics in the list view

A forum page carries its subforums and its topics in tables of their own,
each with a header. Rebuilding the page into one stream of cards dissolves
that separation: both kinds render identically, so where the subforums end
and the topics begin was no longer visible, and a single card gave no hint
whether tapping it opens a forum or a thread.

A header naming the kind and its count is therefore inserted wherever the
kind changes, and forum cards are tinted and edged. Directly below a
category heading the header is suppressed — on the forum root that heading
already introduces the forums under it, and repeating the kind there adds
a strip per category for nothing.

Counting requires knowing how many cards a block actually yields, which is
not the number of rows: rows that turn out to be layout or fail conversion
drop out. The cards are therefore built completely before any of them is
emitted.

README.md

@@ -27,6 +27,13 @@ card; category headers become section headings. Rows are recognised by phpBB's
 own `forumlink` / `forumdesc` classes, and failing that by their shape — two
 purely numeric columns are only ever a forum's topic and post count.
 
+Where forums and topics end up in the same stream of cards, a header strip
+naming the kind and its count (`FOREN 4`, `THEMEN 3`) is inserted wherever the
+kind changes, and forum cards additionally carry a coloured left edge. The
+original page separates the two by putting them in tables of their own; that
+boundary is otherwise lost. The strip is left out directly below
+a category heading, which already introduces the forums under it.
+
 The thread navigation bar holds first / previous / next / last and a page
 indicator. Tapping the indicator opens a sheet with a number field and a grid of
 all pages. Targets are computed as `start = (page - 1) × per_page` rather than
@@ -199,6 +206,13 @@ Zwischenüberschriften. Erkannt werden die Zeilen an phpBBs eigenen Klassen
 `forumlink` / `forumdesc` und ersatzweise an ihrer Form — zwei rein numerische
 Spalten hat nur eine Forenzeile mit Themen- und Beitragszahl.
 
+Stehen Foren und Themen im selben Kartenstrom, wird bei jedem Wechsel ein
+Gruppenkopf mit Art und Anzahl eingezogen (`FOREN 4`, `THEMEN 3`); Forenkarten
+tragen zusätzlich eine farbige linke Kante. Die Originalseite trennt beide über
+eigene Tabellen mit eigenem Kopf — diese Grenze geht sonst verloren.
+Direkt unter einer Kategorieüberschrift entfällt der
+Gruppenkopf, weil die Überschrift die Foren darunter schon einleitet.
+
 Die Navigationsleiste im Thread enthält erste / vorherige / nächste / letzte
 Seite und eine Positionsanzeige. Ein Tipp darauf öffnet ein Sheet mit
 Zahlenfeld und einem Raster aller Seiten. Die Ziele werden als