Add offline status backups and debounced persistence

[aliorhun]

Summary

Implements items 10–12 in one change set:

• show a compact online/offline indicator in the editor status bar
• update the indicator immediately from browser online / offline events
• add a Backup modal beside Export
• export all notebooks, the active tab and basic UI preferences as one versioned JSON file
• validate MathNote backup format and notebook records before restore
• confirm before replacing the current notebook collection
• restore language, zoom and sidebar width together with notebook data
• debounce editor persistence to localStorage by 300 ms
• keep structural actions (new file, tab switch, delete, restore) synchronous
• flush pending edits when the page is hidden or unloaded

Backup format

Backups use:

{
  "format": "mathnote-backup",
  "version": 1,
  "exportedAt": "...",
  "files": [],
  "activeIndex": 0,
  "preferences": {}
}

Restore only accepts the supported format/version and validates every notebook name/content pair before applying changes.

Persistence behavior

Normal typing now coalesces repeated synchronous localStorage.setItem() calls into a single write after 300 ms. Actions that change notebook structure still write immediately to avoid losing state during navigation or tab changes.