feat: smart auto-cleanup, resilient undo, and auto-K threshold

[Harsh16gupta]

This PR handles automatic cleanup of empty notebooks after note moves, improves the undo mechanism to recreate deleted folders, and tweaks auto-K for larger datasets.

Key changes:

• Auto-delete original notebooks that become empty after notes are moved during apply.
• Resilient undo: store original folder titles/parent IDs in the changelog and batch-check folder existence on undo. If a folder was auto-cleaned, recreate it by title and cache the ID so we don't spam duplicate folder creation calls.
• Removed manual cleanup UI button, tools menu command, settings, and webview cleanup state.
• Set min K to 3 for datasets with 20+ notes to avoid splitting larger sets into just 2 categories.
• Bumped version to 0.1.5.

Tested locally

[Harsh16gupta]

Have been testing the changes locally, and it is working perfectly, so merging it.