Add catalog subject banner feature: Introduce optional header images for subjects in the catalog, update CONTRIBUTING.md with guidelines for adding banners, and modify catalog data structure to support banner URLs. Enhance frontend components and styles to display banners effectively. Update TypeScript types and curriculum structure to accommodate new banner functionality.

data/math-curriculum.json

@@ -5,7 +5,9 @@
   "graph": {
     "edges": [
       { "from": "counting-and-numbers", "to": "basic-math-notation" },
-      { "from": "basic-math-notation", "to": "directed-numbers-rates" },
+      { "from": "basic-math-notation", "to": "properties-of-operations" },
+      { "from": "properties-of-operations", "to": "written-calculation" },
+      { "from": "written-calculation", "to": "directed-numbers-rates" },
       { "from": "directed-numbers-rates", "to": "polynomials-expansion-factorisation" },
       { "from": "polynomials-expansion-factorisation", "to": "algebraic-fractions-formulas" },
       { "from": "polynomials-expansion-factorisation", "to": "linear-equations-two-unknowns" },
@@ -65,6 +67,20 @@
           "status": "live",
           "curriculums": ["Foundation"]
         },
+        {
+          "slug": "properties-of-operations",
+          "title": "Properties of operations",
+          "description": "See why order, grouping, and distribution can change how you write a sum or product without changing the result — with interactive area and dot models.",
+          "status": "live",
+          "curriculums": ["Foundation"]
+        },
+        {
+          "slug": "written-calculation",
+          "title": "Written calculation",
+          "description": "Add, subtract, multiply, and divide whole numbers using vertical written form: align columns, regroup, and read the result.",
+          "status": "live",
+          "curriculums": ["Foundation"]
+        },
         {
           "slug": "directed-numbers-rates",
           "title": "Directed numbers, rates & ratios",