refactor(student): improve onboarding validation with React Hook Form

[Tanushree-RD]

Summary

This PR rebases the React Hook Form migration onto the latest main and preserves the existing onboarding architecture.

Changes

• Preserved the centralized onboarding schema
• Preserved API-based onboarding submission
• Preserved existing validation and error handling
• Added forward validation when navigating via the step indicator
• Added unit tests for the onboarding schema

Conflict Resolution

• Kept the latest dependency versions from main
• Removed the obsolete local onboarding schema
• Preserved the existing API mutation and submission flow
• Merged the step indicator validation improvements into the current implementation

Validation

• ✅ turbo run lint
• ✅ turbo run check-types
• ✅ turbo run test
• ✅ turbo run build

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