feat: add proper form validation with Zod schemas to all forms (Closes #477)

[waterWang]

Description

Adds proper form validation with Zod schemas to all forms, as specified in #477.

Changes

• New frontend/src/schemas.ts: Centralized Zod schemas for EmployeeEntry and PayrollScheduler forms
• EmployeeEntry: Added Zod validation (fullName, email, walletAddress, role, currency) with inline error messages, blur validation, and submit button disabled while invalid
• PayrollScheduler: Added Zod validation (employeeName, amount, frequency, startDate) with inline error messages, blur validation, and submit button disabled while invalid

Acceptance Criteria

• Every form field has a Zod schema
• Validation runs on blur and on submit
• Error messages appear inline below each field
• Submit button disabled while form is invalid

Notes

• Uses only zod (already in dependencies) — no new dependencies added
• No react-hook-form required — implemented with plain Zod + React state
• Settings page already has no form (only a language selector dropdown), so no changes needed there