Add validation for search page URL parameters

Introduce Zod-based validation for search parameters to improve UX
and avoid unnecessary API calls with invalid inputs:

- Add search-params.ts with schema validation for q, ai, a, p, s params
- Validate author parameter format (Hive username rules)
- Enforce max length limits on search queries (500 chars)
- Validate sort parameter against allowed values

Invalid parameters are silently omitted rather than causing errors,
providing graceful degradation for malformed URLs.