Use MathDisplay for problem header in flowchart walker

- Render problem header with MathDisplay component for consistent math formatting
- Fix type error in validateCheckpoint for two-numbers input

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/src/components/flowchart/FlowchartWalker.tsx

@@ -617,10 +617,11 @@ export function FlowchartWalker({
         )}
 
         {/* Problem display */}
-        <div data-testid="problem-header" className={css({ fontSize: 'sm' })}>
-          <span className={css({ color: { base: 'gray.500', _dark: 'gray.500' } })}>
-            {problemDisplay}
-          </span>
+        <div
+          data-testid="problem-header"
+          className={css({ color: { base: 'gray.500', _dark: 'gray.500' } })}
+        >
+          <MathDisplay expression={problemDisplay} size="sm" />
         </div>
 
         {/* Change problem link (when not at start) */}

apps/web/src/lib/flowcharts/loader.ts

@@ -298,14 +298,12 @@ export function validateCheckpoint(
   if (!node || node.definition.type !== 'checkpoint') return null
 
   const def = node.definition
-  const context: EvalContext = {
-    ...createContextFromState(state),
-    input: userInput,
-  }
 
   try {
     // Handle two-numbers input type with array of expected expressions
     if (def.inputType === 'two-numbers' && Array.isArray(def.expected)) {
+      // For two-numbers, we don't use `input` in expressions - we validate against problem values
+      const context: EvalContext = createContextFromState(state)
       const expected1 = evaluate(def.expected[0], context) as number
       const expected2 = evaluate(def.expected[1], context) as number
       const expectedArray: [number, number] = [expected1, expected2]
@@ -323,6 +321,10 @@ export function validateCheckpoint(
     }
 
     // Original single-value validation
+    const context: EvalContext = {
+      ...createContextFromState(state),
+      input: userInput as ProblemValue,
+    }
     const expected = evaluate(def.expected as string, context)
     const tolerance = def.tolerance ?? 0