fix(admin): neutralize formulas in CSV exports

[rissrice2105-agent]

Summary

• neutralize formula-like user strings before CSV serialization
• preserve numeric cells as numeric values
• retain correct escaping for commas, quotes, CR, and LF
• add regression coverage for direct and whitespace-prefixed formula markers

Closes #246

Validation

• �itest run src/app/admin/users/UsersContent.test.tsx src/app/api/admin/users/route.test.ts src/lib/stats/admin-user-stats.test.ts - 23 passed
• full �itest run - 3,775 passed, 12 skipped
• focused ESLint - passed
• sc --noEmit - passed
• git diff --check - passed