feat: support nested (sub-)folders

[Wikid82]

Summary

• Adds an optional parentId to BookmarkFolder, capped at MAX_FOLDER_DEPTH = 3 levels (root-level folders are depth 1) — depth limit and DnD-reparenting scope were confirmed with the maintainer before implementing.
• createSubfolder (context menu on a folder): creates a folder directly under it, skipping a parent picker since the parent is implied by the click. Blocked with a warning if it would exceed the depth cap.
• moveFolderToParent (context menu on a folder): reparents an existing folder via a picker (pickParentFolder) that excludes the folder itself, its descendants, and any parent that would exceed the depth cap.
• deleteFolder now promotes direct child folders to the root (not deleted) the same way it already does for direct bookmarks.
• getChildren recurses through the hierarchy; FolderGroupItem carries its direct child folders and shows a subfolder count alongside the bookmark count. List view shows a full breadcrumb path (Backend › Auth Service) instead of just the immediate folder name.
• Drag-and-drop: dropping a folder onto another folder now nests it as that folder's last child. VS Code's tree DnD API can't distinguish "dropped near" from "dropped directly on" a target, so this collides with the sibling-reordering shipped in Reorder bookmarks and folders via drag-and-drop #38 — nesting wins since it's the primary reason to drag a folder onto another folder (confirmed this tradeoff with the maintainer). Reordering root-level folders now happens by dropping on empty tree space, which still appends to the end. Nesting that would exceed the depth cap or create a cycle is refused with a warning message.
• pickFolder (for bookmarks) now renders breadcrumb labels too, for disambiguating same-named folders at different nesting levels — a no-op change for anyone not using nesting, since a root folder's breadcrumb is just its name.

Closes #41

Test plan

• npm run lint
• npm run test:coverage — 100% statements/lines, 95.71% branches
• npm run build
• npm run test:e2e — 8/8 passing against a real VS Code instance, including nested creation/recursion and a real drag-and-drop reparent
• npm audit --audit-level=high — 0 vulnerabilities
• scripts/local-patch-report.sh — 100% patch coverage (112/112)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.58120% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
extension.ts	96.58%	0 Missing and 4 partials ⚠️

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