fix(worksheets): increase color visibility for difficulty presets

Changed from .50 backgrounds to .100 for better visibility
Changed early learner from cyan to blue for better distinction
Changed borders from .400 to .500 for more prominent accents
Changed text from .700 to .800 for better contrast

The cyan.50 background was nearly invisible against white.
New colors are still subtle but clearly visible.

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

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

apps/web/src/app/create/worksheets/addition/difficultyProfiles.ts

@@ -368,11 +368,11 @@ export interface DifficultyProfile {
  * Subtle progression from green (easy) to red (hard)
  */
 export const DIFFICULTY_COLORS = {
-  beginner: { bg: 'green.50', border: 'green.400', text: 'green.700' },
-  earlyLearner: { bg: 'cyan.50', border: 'cyan.400', text: 'cyan.700' },
-  intermediate: { bg: 'yellow.50', border: 'yellow.400', text: 'yellow.700' },
-  advanced: { bg: 'orange.50', border: 'orange.400', text: 'orange.700' },
-  expert: { bg: 'red.50', border: 'red.400', text: 'red.700' },
+  beginner: { bg: 'green.100', border: 'green.500', text: 'green.800' },
+  earlyLearner: { bg: 'blue.100', border: 'blue.500', text: 'blue.800' },
+  intermediate: { bg: 'yellow.100', border: 'yellow.500', text: 'yellow.800' },
+  advanced: { bg: 'orange.100', border: 'orange.500', text: 'orange.800' },
+  expert: { bg: 'red.100', border: 'red.500', text: 'red.800' },
 } as const
 
 /**