feat: reorder bookmarks and folders via drag-and-drop

[Wikid82]

Summary

• Adds an optional order field to Bookmark/BookmarkFolder and a sortByOrder helper that sorts by it when set, falling back to the existing createdAt/name ordering for anything not yet touched by a drag.
• Wires BookmarksTreeProvider up as a vscode.TreeDragAndDropController (registered via dragAndDropController on createTreeView) to support:
  • Reordering bookmarks within a folder/list
  • Reordering folders themselves
  • Dragging a bookmark onto a folder to move it (alternative to "Move to Folder...")
• List view reorders as one flat scope without reassigning folderId; tree view reassigns folderId when a bookmark is dropped across folders.

Closes #38

Test plan

• npm run lint
• npm run test:coverage — 100% statements/lines, 94.11% branches
• npm run build
• scripts/local-patch-report.sh — 100% patch coverage (69/69)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.65217% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
extension.ts	95.65%	0 Missing and 3 partials ⚠️

📢 Thoughts on this report? Let us know!