feat: CSS Grid editor with OCR-measured column widths and row heights

Backend: add layout_metrics (avg_row_height_px, font_size_suggestion_px)
to build-grid response for faithful grid reconstruction.

Frontend: rewrite GridTable from HTML <table> to CSS Grid layout.
Column widths are now proportional to the OCR-measured x_min/x_max
positions. Row heights use the average content row height from the
scan. Column and row resize via drag handles (Excel-like).

Font: add Noto Sans (supports IPA characters) via next/font/google.

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

admin-lehrer/components/grid-editor/GridEditor.tsx

@@ -195,6 +195,7 @@ export function GridEditor({ sessionId, onNext }: GridEditorProps) {
           >
             <GridTable
               zone={zone}
+              layoutMetrics={grid.layout_metrics}
               selectedCell={selectedCell}
               onSelectCell={setSelectedCell}
               onCellTextChange={updateCellText}