[Aug 17] Build local payroll input and validation

[Avinash1286]

Implement the privacy-safe three-row payroll input path.

• Manual entry and local CSV parsing
• Decimal-to-smallest-unit conversion
• Starknet address validation
• Duplicate recipient rejection
• Aggregate calculation
• Demo CSV fixture
• Verify no plaintext row leaves the browser

Exit criterion: a valid three-row payroll imports with no server request.