feat(backend): add Zod server-side validation for task creation (#66)

[julianvillalba688]

Description

This PR addresses the security vulnerability where task creation relied exclusively on client-side validation. I have implemented a rigorous server-side validation layer inside the Next.js API route using Zod to sanitize required fields, enforce valid reward limits, and ensure future-date deadlines before any interaction with the ledger client happens.

Closes #66

Changes Made

1. Zod Validation Schema (src/lib/taskValidation.ts):
   • Engineered a strict taskSchema enforcing field types, minimum string lengths (title >= 5, description >= 10), explicit positive boundary checks for rewards (> 0), and dynamic future timestamp constraints for deadlines.
2. API Endpoint Pipeline (src/app/api/tasks/route.ts):
   • Built a secure POST endpoint that safe-parses incoming client JSON against the Zod schema.
   • Structured error normalization to catch failures and format field-specific feedback using .issues, responding with a clean 400 Bad Request payload.
3. Robust Unit Tests (src/lib/taskValidation.test.ts):
   • Formulated an exhaustive testing matrix via Vitest validating: the happy path, fields omitted/missing, out-of-bounds zero/negative rewards, and elapsed deadlines.
4. Build Fix (src/components/Navbar.tsx):
   • Patched an inherited compilation barrier regarding duplicated attributes in the Navbar layout, unblocking successful production bundles.

Verification Matrix

• Vitest Pipeline: All validation tests execute and pass cleanly.
• Production Build: Next.js optimized production build succeeds with zero linter or TypeScript type check alerts.