feat: add Excel-like grid editor for OCR overlay (Kombi mode step 6)

Backend: new grid_editor_api.py with build-grid endpoint that detects
bordered boxes, splits page into zones, clusters columns/rows per zone
from Kombi word positions. New DB column grid_editor_result JSONB.

Frontend: GridEditor component with editable HTML tables per zone,
column bold toggle, header row toggle, undo/redo, keyboard navigation
(Tab/Enter/Arrow), image overlay verification, and save/load.

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

admin-lehrer/components/grid-editor/index.ts

@@ -0,0 +1,6 @@
+export { GridEditor } from './GridEditor'
+export { GridTable } from './GridTable'
+export { GridToolbar } from './GridToolbar'
+export { GridImageOverlay } from './GridImageOverlay'
+export { useGridEditor } from './useGridEditor'
+export type * from './types'