fix: add 'auto' to RuleMode type to prevent undefined display values

The 'auto' display rule value was added to config schemas but not to the
RuleMode type definition, causing evaluateRule to return undefined when
encountering 'auto' values that weren't properly resolved.

Changes:
- Add 'auto' to RuleMode type definition
- Add 'auto' case in evaluateRule that logs error and defaults to 'always'
- Prevents 'undefined' from reaching Typst generator

This is a defensive fix - 'auto' should be resolved in validation.ts
before reaching the display rule evaluator, but this prevents crashes
if resolution fails for any reason.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/web/src/app/create/worksheets/displayRules.ts

@@ -5,6 +5,7 @@ import type { ProblemMeta, SubtractionProblemMeta } from './problemAnalysis'
 export type AnyProblemMeta = ProblemMeta | SubtractionProblemMeta
 
 export type RuleMode =
+  | 'auto' // Defer to mastery progression (should be resolved before rendering)
   | 'always' // Always show this display option
   | 'never' // Never show this display option
   | 'whenRegrouping' // Show when problem requires any regrouping
@@ -39,6 +40,14 @@ export interface ResolvedDisplayOptions {
  */
 export function evaluateRule(mode: RuleMode, problem: AnyProblemMeta): boolean {
   switch (mode) {
+    case 'auto':
+      // 'auto' should have been resolved to a concrete value in validation
+      // If it reaches here, something went wrong - default to 'always' to avoid breaking
+      console.error(
+        '[evaluateRule] BUG: "auto" mode should have been resolved before rendering. Defaulting to "always".'
+      )
+      return true
+
     case 'always':
       return true