fix(#240): rename segmentation column heading to "Text markieren"

"Rahmen einzeichnen" assumed familiarity with the segmentation concept;
"Text markieren" is self-explanatory for new contributors.

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

frontend/messages/en.json

@@ -559,7 +559,7 @@
 	"training_chip_kurrent": "Kurrent recognition",
 	"training_chip_segmentation": "Segmentation",
 	"mission_control_heading": "What needs attention?",
-	"mission_control_segmentation_heading": "Draw regions",
+	"mission_control_segmentation_heading": "Mark text",
 	"mission_control_segmentation_description": "Mark text areas — no prior knowledge needed",
 	"mission_control_seg_skill_pill": "✓ No prior knowledge",
 	"mission_control_segmentation_empty": "All documents already have segmentation blocks.",