Hot take: in 2026, the distinction between "full-stack" and "mobile" matters less than it used to, because AI coding agents handle most of the platform-specific boilerplate.
What actually moves the needle for a senior frontend dev: understanding systems design, API contracts, and state management patterns that transfer across any platform. If you can architect a clean data flow, Claude Code or Cursor can help you implement it in React, Swift, or Flutter.
My suggestion: go full-stack. The backend knowledge compounds — understanding databases, auth, and deployment gives you the ability to ship complete products independently. Mobile-only devs often get bottlenecked by backend dependencies. Full-stack devs ship.