Exclude admin and owner browser traffic from GA4

[Aprasaks]

Summary

Exclude Dechive admin and owner browser visits from GA4 tracking so the private analytics dashboard is not polluted by maintenance traffic.

Scope

• Move GA script loading into a client component
• Skip GA loading on /admin routes
• Add an admin dashboard toggle that stores a browser-level GA opt-out flag in localStorage
• Keep public page tracking active for normal visitors

Validation

• npm run lint
• npm run build