feat(editor): add timeout modal for file loading issues and enhance error handling

[NicoTuxx]

Testing the Timeout Modal

To test the scenario where the timeout modal appears but the file eventually loads:

Temporarily make these changes in FileEditor.vue:

1. Reduce the timeout threshold (around line 445):

const LOADING_TIMEOUT_MS = 3000; // Reduced from 30000 for testing

2. Add a simulated delay to the onReady event (around line 369):

onReady: (): void => {
  // Simulate slow file loading to trigger timeout modal
  const SIMULATED_DELAY = 5000; // 5 seconds
  window.setTimeout(() => {
    window.electronAPI.log('info', 'CryptPad editor is ready and document loaded successfully');
    fileLoaded.value = true;
  }, SIMULATED_DELAY);
},

Expected behavior:

1. Open a CryptPad-supported file (e.g., .xlsx, .md)
2. After 3 seconds, a timeout modal should appear asking if you want to wait or go back
3. Click "Wait" button
4. After ~2 more seconds, the file should finish loading
5. Click "Close" to dismiss the modal and start editing (it should not redirect you)

This tests that the timeout handling properly continues waiting when the user chooses to wait, correctly detects when the file finally loads, and updates the button accordingly.

Remember to remove both testing modifications before committing.