feat: add ability to create new collections directly from popup

[danilosekulovikj]

Summary

Allows users to create new collections directly from the extension popup and options page, significantly reducing friction when saving links.

Features and improvements

• Deferred Creation: Lazily creates the collection upon saving the bookmark to prevent orphaned empty collections if the user cancels.
• UX Improvements:
  • Implements a pinned footer inside the collection dropdown for creating a new collection based on the search input.
  • Displays a clear, centered 'Create' button in the empty state when no existing collections match the search query.
• Validation & Safety:
  • Uses trimmed, case-insensitive checks to prevent creating duplicate collections or whitespace-only names.
  • Updated the underlying cmdk filter function to .trim() search input, ensuring that typing trailing spaces (e.g., "Test ") doesn't incorrectly hide existing collections.
• Error Handling: Adds specific error handling to distinguish between link save failures and collection creation failures.
• Code Quality: Refactors the dropdown collection list to eliminate ~80 lines of code duplication across rendering paths.
• Polish: Fixes dark mode styling (bg-neutral-900) for the dropdown overlay and fixed a useEffect linter warning in BookmarkForm.tsx.

How to Test

1. Open the extension popup.
2. Open the Collection dropdown and start typing a collection name that doesn't exist.
3. Click the pinned "Create [name]" footer or the centered empty state button.
4. Save the link and verify the link appears in the newly created collection in your Linkwarden instance.
5. Check edge cases: try creating a collection with trailing spaces, or click the "X" close button after preparing a new collection to verify the form state resets safely.