fix(ocr-pipeline): preserve sub-column data in vocab table display

Three fixes for sub-columns disappearing at end of streaming:

1. Backend: add column_marker mapping in _cells_to_vocab_entries()
   so marker text is included in vocab entries (not silently dropped)

2. Frontend types: add source_page and bbox_ref to WordEntry interface

3. Frontend table: show page_ref column (Seite) in vocab table when
   entries have source_page data, instead of only EN/DE/Example

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

admin-lehrer/app/(admin)/ai/ocr-pipeline/types.ts

@@ -180,11 +180,13 @@ export interface WordEntry {
   english: string
   german: string
   example: string
+  source_page?: string
   confidence: number
   bbox: WordBbox
   bbox_en: WordBbox | null
   bbox_de: WordBbox | null
   bbox_ex: WordBbox | null
+  bbox_ref?: WordBbox | null
   status?: 'pending' | 'confirmed' | 'edited' | 'skipped'
 }