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/i18n/keep.txt

@@ -0,0 +1,46 @@
+# Strings utils/i18n_sync.py stores as themselves instead of translating.
+# One per line; blank lines and lines starting with # are ignored.
+
+Baserow
+Big Blue Button
+Bluesky
+Buy me a Coffee
+Discourse
+Duolingo
+Eversports
+Facebook
+Friendica
+Garmin
+Gitea
+GitHub
+GitHub Sponsors
+Infinito.Nexus
+Instagram
+Keycloak
+LDAP
+LinkedIn
+Mailu
+Mastodon
+Matomo
+Matrix
+Nextcloud
+Open Project
+Patreon
+PayPal
+Peertube
+Pixelfed
+Signal
+Snipe IT
+Spotify
+Taiga
+Telegram
+Twitter
+WhatsApp
+XING
+YouTube
+Yourls
+Zoom
+freelancermap.de
+malt
+phpMyAdmin
+upwork.com