fix(web): add char limit validation to custom question fields

[guangshinhaha]

Summary

• Add 120-char limit on custom question text and 250-char limit on helper text (description), matching PG's validation
• Add maxLength + character counters to QuestionBuilder inputs (same pattern as title/venue fields)
• Add form-level validation in isCreatePostFormValid so the submit button disables when limits are exceeded
• Add unit tests for boundary conditions (at limit, over limit)

Closes #45

Note: The 120/250 limits are based on the PG screenshot in the issue. Since we run in mock mode, these have not been verified against actual PGW FE/BE validation. Cross-check with the PGW codebase before merging.

Test plan

• Verify question text input stops at 120 characters and shows counter
• Verify helper text textarea stops at 250 characters and shows counter
• Verify submit button disables if limits are somehow exceeded
• Cross-check char limits against PGW FE/BE codebase (mock mode — can't test live)
• pnpm test passes (5 new validation tests added)

🤖 Generated with Claude Code