feat: enable nested folder/collection creation via context menu

[Copilot]

The folder right-click menu had no "New Folder" or "New Collection" options, making it impossible to create items inside existing folders through the UI — despite the backend fully supporting nested paths via parentRelPath.

Changes

• ItemContextMenu — Added optional onAddFolder and onAddCollection props; renders "New Folder" / "New Collection" buttons (with FolderPlus / FileJson icons) at the top of the menu when provided.

• FolderItem — Added onStartCreate(parentId, kind, parentRelPath) prop; wires the context menu to trigger inline creation scoped to the folder's own relPath; propagates onStartCreate to recursively rendered children.

• TreeItem — Threads onStartCreate through to FolderItem.

• CollectionPanel — Adds handleStartCreate which auto-expands the target folder before setting creating state, so the inline input row is immediately visible.

• Icon fix — InlineCreateRow for collections was incorrectly rendering FolderClosed; corrected to FileJson at both root and nested levels.

Right-clicking any folder now exposes:

┌─────────────────┐
│ 📁 New Folder   │  ← creates subfolder via createCollectionFolder(ws, folder.relPath, name)
│ {} New Collection│  ← creates collection inside folder
│ Open Docs       │
│ Rename          │
│ Delete          │
└─────────────────┘

Original prompt

I'd like to further improve the folder structure for the app. What i want to be ablet o do is folder within folders. I want an easy, logical way to create folders within folders

Created from VS Code.