feat(search): restyle highlights to navy underline and add snippet labels

Switch search match highlights from bordered mint chips to a plain navy
underline (decoration-brand-navy). Add visible "Inhalt" / "Content" /
"Contenido" label before the transcription snippet, matching the style
of the Von/An sender-receiver labels.

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

frontend/messages/en.json

@@ -78,6 +78,8 @@
 	"docs_empty_btn_clear": "Clear all filters",
 	"docs_list_from": "From",
 	"docs_list_to": "To",
+	"docs_list_content": "Content",
+	"docs_list_summary": "Summary",
 	"docs_list_unknown": "Unknown",
 	"docs_group_undated": "Undated",
 	"docs_group_unknown": "Unknown",