fix: set approval log fields based on workflow state

[ravibharathi656]

Summary:

• Added shg_approval_by and shg_approved_on fields to NominationForm to capture SHG-level approval.
• Refactored set_approval_log() to use a state-to-prefix mapping (SHG Proposed, VO Approved, CLF Approved), eliminating repetitive if/elif blocks and making it easy to extend with new states.
• Replaced raw current_user (email) with get_fullname() for more readable approval records.
• Moved approval log logic from on_update to before_save to avoid duplicate writes.