Use env-driven API base URL (Vite env) instead of hardcoded

[sugan0tech]

Summary
Use a Vite environment variable for API base URL instead of a hardcoded localhost constant.

Context
src/utils/api.ts defines BASE_URL = "http://localhost:8080/api". This blocks environment-specific configuration for staging/production.

Tasks

• Read base URL from import.meta.env.VITE_API_BASE_URL with a sensible dev default.
• Add .env.example documenting VITE_API_BASE_URL.
• Update README with configuration instructions for local dev and deployments.
• Verify cookie/security options remain appropriate (e.g., secure only in production).

Acceptance Criteria

• Local dev reads value from .env (or defaults to localhost) and works.
• Production build uses the injected value from environment at build time.
• No hardcoded API host remains in the codebase.