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/src/lib/components/TranscriptionEditView.svelte.spec.ts

@@ -40,6 +40,7 @@ function renderView(overrides: Record<string, unknown> = {}, service = createCon
 				onBlockFocus: vi.fn(),
 				onSaveBlock: vi.fn(),
 				onDeleteBlock: vi.fn(),
+				onReviewToggle: vi.fn(),
 				...overrides
 			},
 			context: new Map([[CONFIRM_KEY, service]])