fix(mastodon): improve autocompletion scan performance and UX

[kastwey]

• Use dict instead of list for duplicate checking (O(1) vs O(n) per lookup),
  which is faster and more idiomatic for large datasets.
• Add detailed progress dialog with:
  • Account info (followers/following/total counts)
  • Current status and page number.
  • Users processed counter.
  • Visual progress bar.
  • Cancel button to stop scan gracefully.
• Refactor scan logic into reusable _scan_users() method with ScanType enum.
• Show accurate completion stats (new users vs already in database).
• Fix window close behavior (X button now cancels instead of crashing).
• Add 15 unit tests for storage and scan logic.
• Fix storage.del to handle already-closed connections.