fix: use ± symbol for mixed operator icon consistently

Update SETTING_ICONS to use ± (plus-minus) for mixed mode to match
the operator tab button icon. This ensures consistency across:
- Operator tab navigation button
- Mobile settings button
- Shared worksheet summary

Previously used 🔀 (shuffle) emoji which was inconsistent with the
tab button's ± symbol.

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

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

apps/web/src/app/create/worksheets/utils/settingsSummary.ts

@@ -9,7 +9,7 @@ export const SETTING_ICONS = {
     subtraction: '➖',
     multiplication: '✖️',
     division: '➗',
-    mixed: '🔀',
+    mixed: '±',
   },
   difficulty: {
     smart: '🎯',

apps/web/src/app/worksheets/shared/components/SharedWorksheetSummary.tsx

@@ -143,7 +143,7 @@ export function SharedWorksheetSummary({ config }: SharedWorksheetSummaryProps)
 
       {/* Main Content */}
       <InfoCard
-        icon={config.operator === 'mixed' ? '+−' : SETTING_ICONS.operator[config.operator]}
+        icon={SETTING_ICONS.operator[config.operator]}
         title="What's Being Practiced"
         description={`${operatorLabel} with ${digitRangeText}`}
         isDark={isDark}