docs(vue): add missing ref import in example

[dieutx]

Summary

• add the missing ref import in the Vue TypeScript example
• keep the existing Ref<User | null> type annotation intact

Testing

• not run (README-only change)

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2