Add Ansicht step (Step 12) — read-only page layout preview

New pipeline step showing the reconstructed page with all zones
positioned at their original coordinates:
- Content zones with vocabulary grid cells
- Box zones with colored borders (from structure detection)
- Colspan cells rendered across multiple columns
- Multi-line cells (bullets) with pre-wrap whitespace
- Toggle to overlay original scan image at 15% opacity
- Proportionally scaled to viewport width
- Pure CSS positioning (no canvas/Fabric.js)

Pipeline: 14 steps (0-13), Ground Truth moved to Step 13.
Added colspan field to GridEditorCell type.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -126,6 +126,8 @@ export interface GridEditorCell {
   is_bold: boolean
   /** Manual color override: hex string or null to clear. */
   color_override?: string | null
+  /** Number of columns this cell spans (merged cell). Default 1. */
+  colspan?: number
 }
 
 /** Layout dividers for the visual column/margin editor on the original image. */