Improve email verification flow and error handling

- Send verification email before creating user to prevent orphaned accounts
- Detect SMTP authentication errors and fail fast without retries
- Add field-level validation errors for better frontend UX
- Configure docker-compose with explicit environment variables
- Document dollar sign escaping in .env.example (use $$)
- Implement welcome email on successful verification
- Clean up unnecessary comments

internal/infrastructure/http/dto.go

@@ -85,5 +85,6 @@ type HabitEntriesResponse struct {
 }
 
 type ErrorResponse struct {
-	Error string `json:"error"`
+	Error string  `json:"error"`
+	Field *string `json:"field,omitempty"`
 }