Add environment-based API base URL configuration

[Dannyswiss1]

client/src/lib/api.ts is entirely in-memory today with no concept of a backend URL. Add VITE_API_BASE_URL
support (via .env / .env.example) and a thin fetch wrapper, so a future PR that replaces the mock functions'
internals with real fetch calls has a documented, consistent place to point at locka-api.

Acceptance criteria

• .env.example added documenting VITE_API_BASE_URL and any other needed vars
• A client/src/lib/http.ts fetch wrapper exists (base URL, JSON handling, error normalization) even if unused
  by the mock layer yet
• client/README.md documents how to point the client at a real backend