Folder duplicate: needs a server-side recursive copy endpoint

[Rasbandit]

Duplicating a folder is the one action missing from the folder context menu (added in #1121). It was deliberately left out rather than shipped fragile.

Why it isn't just a frontend change

useDuplicateNote is a single-note operation: GET the source, then one crdt_create_with_content for the copy. There is no folder-copy endpoint, so duplicating a folder client-side would mean:

1. enumerate every descendant folder (walk /api/folders by path prefix),
2. fetch each folder's note list,
3. create the destination folder tree,
4. duplicate every note individually.

That's N round trips with no atomicity. A failure partway leaves a half-copied folder in the vault — a bad state for a knowledge base, and one the user then has to clean up by hand. Retrying isn't safe either, since the already-copied notes would collide.

Proposed

A server-side endpoint that performs the recursive copy in one transaction, e.g.:

POST /folders/duplicate  { src_path, new_path }

Server-side it can reuse the existing name-collision policy (collideBump's "<name> 1" shape) for the destination folder name, and mint note ids the same way the CRDT create path does so the copies are real live docs rather than detached rows.

Frontend follow-up once it exists

• Add { id: "duplicate", label: "Duplicate" } back to FOLDER_ACTIONS in frontend/src/viewer/tree-actions/action-list.ts (the icon is already mapped in ACTION_ICONS).
• Handle it in handleActionPick in frontend/src/viewer/folder-tree.tsx, alongside the existing note-duplicate case.
• Derived folders (syn: ids) have no backend id, so this must be path-addressed like rename/delete/move — see the path-vs-id split in partition().

Acceptance

• Duplicating a folder copies every descendant note and subfolder, or nothing at all.
• The copy's name follows the existing collision policy.
• Works for derived folders (those /api/folders returns with a null id), not just explicit markers.

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https://claude.ai/code/session_013fXPYtAKeUBLKjW9QmVRpZ