Add email validation to tenant email field

Added conditional required validation and format validation for tenant email.

Changes:
- Tenant email is now required when autoTenantNotification is enabled
- Added email format validation using Zod's built-in email validator
- Email field validates format even when autoTenantNotification is off (if not empty)
- Added localization strings for validation errors (Croatian/English)

Validation logic:
- Format validation: Always checks if email is valid format (when not empty)
- Required validation: Email required when autoTenantNotification is true
- Error messages: "email address is invalid" for format, "tenant email is missing" for required

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

messages/en.json

@@ -145,6 +145,8 @@
       "location-name-required": "Relaestate name is required",
       "tenant-first-name-required": "tenant first name is missing",
       "tenant-last-name-required": "tenant last name is missing",
+      "tenant-email-required": "tenant email is missing",
+      "tenant-email-invalid": "email address is invalid",
       "validation-failed": "Validation failed. Please check the form and try again."
     }
   },

messages/hr.json

@@ -144,6 +144,8 @@
       "location-name-required": "Ime nekretnine je obavezno",
       "tenant-first-name-required": "nedostaje ime podstanara",
       "tenant-last-name-required": "nedostaje prezime podstanara",
+      "tenant-email-required": "nedostaje email podstanara",
+      "tenant-email-invalid": "email adresa nije ispravna",
       "validation-failed": "Validacija nije uspjela. Molimo provjerite formu i pokušajte ponovno."
     }
   },