refactor(settings): prioritize CalDAV/CardDAV in sync tab, fix heading hierarchy

Open standards (CalDAV, CardDAV, ICS) now appear first in the Sync tab
under a dedicated "CalDAV & CardDAV" section, making self-hosted sync
options prominent. Cloud services (Google, Apple) move to a secondary
"Cloud-Dienste" section. Fixes raw <h2> heading in CalDAV card — now
uses consistent settings-card__title like all other cards.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/locales/de.json

@@ -773,6 +773,8 @@
     "tabSyncCalendar": "Kalender",
     "tabSyncContacts": "Kontakte",
     "sectionContactSync": "Kontakt-Synchronisation",
+    "sectionOpenStandards": "CalDAV & CardDAV",
+    "sectionCloudServices": "Cloud-Dienste",
     "cardavTitle": "CardDAV Kontakte",
     "tabFamily": "Familie",
     "tabApiTokens": "API-Tokens",