docs: the folding channel header in its e2e, and three comments brought up to date

The e2e asserted the channel chrome never moves; it folds once now. The
ui catalog lists scrollFold and progressiveRows, Tabs names the
per-tab memory its callers use, and the welcome note says a removed
language orphans its stored copies too.

backend/app/routes/messages.py

@@ -97,8 +97,9 @@ def _system_partner() -> dict:
 # The welcome's opening line in every language — how a stored copy is recognised. It was rendered
 # once, in the language of the day it was created, so an account that later switched language kept
 # reading it in the old one (E31 S1: the owner's, in English). Recognised here, it is served in the
-# reader's language instead. Derived from WELCOME itself, so only a change to an OPENING line would
-# orphan the copies already stored.
+# reader's language instead. Derived from WELCOME itself, so a change to an OPENING line, or a
+# language taken out of WELCOME, orphans the copies already stored: they are served as stored. The
+# one retired language (German, 2026-07) left none on prod (checked 2026-10-02).
 _WELCOME_OPENINGS = {text.partition("\n")[0] for text in WELCOME.values()}