Fix hardcoded API URLs breaking local development

[Copilot]

All frontend components hardcoded API URLs using https://${process.env.REACT_APP_CODESPACE_NAME}-8000.app.github.dev, causing "undefined" in URLs when the environment variable is unset during local development.

Changes

• Centralized API configuration (src/config/api.js)

  • Priority: REACT_APP_API_BASE_URL → REACT_APP_CODESPACE_NAME → http://localhost:8000
  • Exports getApiUrl(endpoint) helper for consistent URL construction

• Component updates

  • Replaced hardcoded URL templates in Activities, Users, Workouts, Leaderboard, and Teams
  • All API calls now use getApiUrl('/api/endpoint/')

• Documentation

  • Updated .env.example with configuration options
  • Added API_CONFIG.md with usage examples

Example

// Before
const apiUrl = `https://${process.env.REACT_APP_CODESPACE_NAME}-8000.app.github.dev/api/users/`;

// After
import { getApiUrl } from '../config/api';
const apiUrl = getApiUrl('/api/users/');

Environment variable priority enables custom backend URLs via REACT_APP_API_BASE_URL while maintaining Codespaces compatibility and providing localhost fallback.

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