feat(frontend): add expandable text component for long fields

Adds ExpandableText.svelte which clamps text to 10 lines and shows a
toggle button only when the content actually overflows. Applied to the
summary and transcription fields on the document detail page.

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

frontend/messages/en.json

@@ -238,5 +238,7 @@
 	"admin_system_backfill_heading": "Backfill history data",
 	"admin_system_backfill_description": "Creates an initial history entry for all documents that do not have one yet (e.g. imported documents). This ensures that future edits only highlight actually changed fields.",
 	"admin_system_backfill_btn": "Backfill now",
-	"admin_system_backfill_success": "{count} documents were backfilled."
+	"admin_system_backfill_success": "{count} documents were backfilled.",
+	"comp_expandable_show_more": "Show more",
+	"comp_expandable_show_less": "Show less"
 }