Add centralized API configuration with localhost fallback

[Copilot]

Hard-coded API URLs using process.env.REACT_APP_CODESPACE_NAME broke local development with https://undefined-8000.app.github.dev/... when the env var was unset.

Changes

• Created src/config/api.js: Centralized configuration with priority-based resolution:

  1. REACT_APP_API_BASE_URL (explicit URL, recommended)
  2. REACT_APP_CODESPACE_NAME (Codespaces, backward compatible)
  3. Default: http://localhost:8000

• Updated all components to use getApiUrl(): Activities, Leaderboard, Teams, Users, Workouts

• Environment configuration: Added REACT_APP_API_BASE_URL to .env and .env.example with documentation

• Documentation: Updated FRONTEND_README.md with configuration examples for local, Codespaces, and production environments

Usage

import { getApiUrl } from './config/api';

const apiUrl = getApiUrl('/api/users/');
// Local dev: http://localhost:8000/api/users/
// Codespaces: https://{name}-8000.app.github.dev/api/users/

App now works locally without requiring environment variables.

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