feat(ui): manual-first OCR workflow — remove full-page auto-segmentation

Drawing annotations is now the primary workflow. OCR only runs on
manually drawn regions (guided mode always). Full-page layout detection
and the useExistingAnnotations checkbox are removed entirely.

- OcrTrigger: guided-only, disabled with hint when no annotations exist
- TranscriptionEditView: empty state shows draw-regions instruction,
  OCR trigger moved out of collapsible and shown inline after block list
- i18n: add ocr_trigger_no_annotations, ocr_section_heading,
  transcription_empty_draw_hint; remove ocr_use_existing_annotations keys

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

frontend/messages/en.json

@@ -479,6 +479,7 @@
 	"scan_collapse": "Collapse scan",
 	"transcription_empty_title": "No transcription yet",
 	"transcription_empty_desc": "Draw regions on the scan and type the text to create a transcription.",
+	"transcription_empty_draw_hint": "Draw regions on the document to start transcribing.",
 	"transcription_panel_close": "Close panel",
 	"person_alias_heading": "Name history",
 	"person_alias_empty": "No name changes recorded yet.",
@@ -530,8 +531,8 @@
 	"ocr_status_analyzing_page": "Analyzing page {current} of {total}…",
 	"ocr_status_done_skipped": "{count} blocks created, {skipped} page(s) skipped",
 	"ocr_status_error": "OCR failed",
-	"ocr_use_existing_annotations": "Annotated regions only",
-	"ocr_use_existing_annotations_hint": "OCR runs only within the already marked regions — no new layout detection.",
+	"ocr_trigger_no_annotations": "Draw regions on the document first.",
+	"ocr_section_heading": "Run OCR",
 	"transcription_block_review": "Mark as reviewed",
 	"transcription_block_unreview": "Unmark as reviewed",
 	"transcription_reviewed_count": "{reviewed} of {total} reviewed",