feat(i18n): translate labels, and keep brand names out of it

name and title were translated at render time but never machine-filled,
on the grounds that no backend tells the menu label "Pictures" from the
brand "Mastodon". That left the visible half of a card in English. They
are filled now, and the two key sets collapse into one.

The brands need somewhere to be named instead. app/i18n/keep.txt lists
them, one per line, and every entry is stored as itself in every target
language: no request, and never over an entry written by hand. --keep adds
one-off strings, --keep-file points elsewhere.

The shipped list holds the 43 product names that appear as name: or title:
in config.sample.yaml. Generic labels — Pictures, Imprint, Settings,
Certificates — are deliberately absent, and so are Cybermaster, Polymath
and Yachtmaster, which read as brand or as job title depending on who is
asking.

Two of these behaviours first shipped unguarded. A test that protected a
string and asserted the hand-written value survived passed either way,
because a run where nothing is missing reports "complete" and never
writes; and nothing exercised main(), so the keep file could stop being
read without a failure. Both are covered.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/utils/i18n.py

@@ -23,9 +23,9 @@ CONTENT_DIR = I18N_DIR / "content"
 SOURCE_LANGUAGE = "en"
 
 
-AUTOFILL_KEYS = frozenset({"description", "text", "warning", "info", "subtitel"})
-
-TRANSLATABLE_KEYS = AUTOFILL_KEYS | frozenset({"name", "title"})
+TRANSLATABLE_KEYS = frozenset(
+    {"description", "info", "name", "subtitel", "text", "title", "warning"}
+)
 
 UI_STRINGS = (
     "Alternatives",