Bug: copyStyleSheetsToWindow() may fail if win.document.head is null

## Bug: `copyStyleSheetsToWindow()` may fail if `win.document.head` is null

**Severity:** Low  
**Files:** `src/js/utils/dom.js`  
**Lines:** 889, 898

### Description
The function appends elements to `win.document.head` without checking if `head` exists. If the target window (e.g., a Picture-in-Picture window) is closing or in an invalid state, `win.document.head` could be `null`, causing an uncaught TypeError.

### Affected code
```javascript
win.document.head.appendChild(style);  // Line 889
// ...
win.document.head.appendChild(link);   // Line 898
```

### Suggested fix
```javascript
const head = win.document.head || win.document.documentElement;
if (head) {
  head.appendChild(style);
}
```

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Bug: copyStyleSheetsToWindow() may fail if win.document.head is null #9186