Integrate Fortune Sheet spreadsheet editor in StepAnsicht

Install @fortune-sheet/react (MIT, v1.0.4) as Excel-like spreadsheet
component. New SpreadsheetView.tsx converts unified grid data to
Fortune Sheet format (celldata, merge config, column/row sizes).

StepAnsicht now has Spreadsheet/Grid toggle:
- Spreadsheet mode: full Fortune Sheet with toolbar (bold, italic,
  color, borders, merge cells, text wrap, undo/redo)
- Grid mode: existing GridTable for quick editing

Box-origin cells get light tinted background in spreadsheet view.
Colspan cells converted to Fortune Sheet merge format.

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

admin-lehrer/package.json

@@ -18,6 +18,8 @@
     "test:all": "vitest run && playwright test --project=chromium"
   },
   "dependencies": {
+    "@fortune-sheet/react": "^1.0.4",
+    "fabric": "^6.0.0",
     "jspdf": "^4.1.0",
     "jszip": "^3.10.1",
     "lucide-react": "^0.468.0",
@@ -26,7 +28,6 @@
     "react-dom": "^18.3.1",
     "reactflow": "^11.11.4",
     "recharts": "^2.15.0",
-    "fabric": "^6.0.0",
     "uuid": "^13.0.0"
   },
   "devDependencies": {