Centralize API URL configuration with localhost fallback

[Copilot]

Hard-coded API URLs using process.env.REACT_APP_CODESPACE_NAME fail in local development, resulting in https://undefined-8000... when the environment variable is unset.

Changes

• New centralized config (src/config/api.js):

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

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

  • Replaced inline URL construction with getApiUrl() imports
  • Removed direct environment variable access from components

// 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/');

This ensures proper API connectivity across local development, GitHub Codespaces, and custom deployments without requiring environment-specific code changes in components.

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