SSR Safe Coding: window/document, localStorage, and TransferState

SSR changes where your code runs. Some code runs on the server first, then in the browser.

## Problem: window/document not available on server

❌ Bad:
```ts
localStorage.getItem('token');
```

## Solution: check platform

```ts
import { inject, PLATFORM_ID } from '@angular/core';
import { isPlatformBrowser } from '@angular/common';

export class TokenService {
  private platformId = inject(PLATFORM_ID);

  getToken() {
    if (!isPlatformBrowser(this.platformId)) return null;
    return localStorage.getItem('token');
  }
}
```

## TransferState (avoid double fetching)

When server fetches data, you can reuse it in the browser without fetching again.

Concept:
- server calls API
- saves response to TransferState
- browser reads it and skips duplicate request

This makes SSR fast and efficient.

> Next: Angular deployment strategies, builds, caching, and versioning.