fix: prevent double date formatting in email templates

- Remove pre-formatting of dates before passing to email processor
- Let email processor handle all date formatting based on recipient language
- Fix Invalid Date errors by passing raw date values instead of formatted strings
- Remove unused formatDate imports and variables

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

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

backend/src/config/validateEnv.js

@@ -34,7 +34,7 @@ function validateEnvironment() {
     if (name === 'JWT_SECRET' && value) {
       // Check for the insecure default value
       if (value === 'your-secret-key') {
-        errors.push(`CRITICAL: JWT_SECRET is set to the insecure default value. Please set a secure secret key.`);
+        errors.push('CRITICAL: JWT_SECRET is set to the insecure default value. Please set a secure secret key.');
       }
       
       // Check minimum length (should be at least 32 characters for security)