Add centralized API configuration with localhost fallback

[Copilot]

Frontend API calls were hard-coded to https://${REACT_APP_CODESPACE_NAME}-8000.app.github.dev, producing invalid URLs (https://undefined-8000...) when the environment variable was unset in local development.

Changes

• Created src/config/api.js: Centralized API base URL resolution with fallback chain:

  1. REACT_APP_API_BASE_URL (explicit override)
  2. REACT_APP_CODESPACE_NAME (Codespaces)
  3. http://localhost:8000 (local default)

• Updated all components: Replaced inline URL construction with getApiUrl() helper in Users, Workouts, Teams, Activities, and Leaderboard components

• Updated .env.example: Documented both configuration options

Example

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

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

The getApiUrl() helper checks for codespaceName !== 'undefined' to prevent malformed URLs.

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