fix(i18n): replace hardcoded "oder" with conv_hero_divider message key

Adds conv_hero_divider to de/en/es messages and uses it in the
CorrespondenzHero divider. Fixes i18n blocker from review.

Refs: #179

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

frontend/messages/en.json

@@ -160,6 +160,7 @@
 	"conv_suggestions_all_label": "All correspondents of {name}",
 	"conv_letters_count": "{count} letters",
 	"conv_empty_search_placeholder": "Search person…",
+	"conv_hero_divider": "or",
 	"conv_empty_recent_label": "Recently opened",
 	"conv_asym_sent": "{count} from {name} →",
 	"conv_asym_received": "{count} from {name} ←",