feat(documents): rename a document folder in place

[Adron]

Summary

APIClient could create and delete document folders but not update one, so fixing a typo in a
folder name meant deleting the folder and recreating it — which took its documents and subfolders
with it. This adds the missing PUT /api/documents/folders/{id} call and a Rename affordance in
the Documents UI, reaching parity with the web's RenameFolderHeader/RenameFolderInput.

Closes #55.

What's included

All in commit 7f79eb0.

• Services/APIClient+Documents.swift (new) — updateDocumentFolder(id:name:parentId:) on
  PUT /api/documents/folders/{id} via putCamel: the route reads plain camelCase keys, so
  the snake_case put would send parent_id and the server would silently ignore the move. New
  feature-extension file rather than growing APIClient.swift, per CLAUDE.md.
• DocumentFolderParentUpdate (.unchanged / .root / .folder) with a hand-written
  encode(to:). The route validates and applies a move whenever the parentId key is present,
  and Swift's synthesized Encodable drops a nil optional — so a rename has to omit the key
  entirely while a move to root needs an explicit null. A plain String? can't express both.
• Views/DocumentsView.swift — Rename in the folder row's context menu and swipe actions,
  in both the root list and the nested DocumentFolderView, as an inline text field swapped in
  for the row. The write is optimistic and rolls the row back to the untouched original if the
  server refuses it.
• Readable refusals — the 400 circular-move rejection and the 409 duplicate-name conflict
  both arrive as APIError.server(_:) carrying server copy that reads well as-is; the alert shows
  it verbatim rather than a generic failure string.
• Services/AppDataStore.swift — updateDocumentFolder(_:) mirrors the saved folder into the
  local tree and persists it, so the rename survives a cold start. It deliberately enqueues no
  sync operation, unlike its createDocumentFolderOffline/deleteDocumentFolderOffline
  neighbours: POST /api/documents/sync answers a folder update with unsupported_operation
  ("Folders support only create and delete"), so an outbox op would be rejected on every push
  and the rename would never persist. Flagged below as a backend follow-up.
• Models/Document.swift — DocumentFolder.renamed(to:), for the optimistic value.
• Tests — 13 request-shape cases in APIClientDocumentsTests: verb and path, parentId
  omitted on rename vs. explicit null on move-to-root vs. an id on move, camelCase keys, the
  absence of any folderId query (folder routes are path-scoped, not query-scoped), and the
  400 / 409 / 401 / missing-folder paths.

Testing

• xcodebuild -scheme InterlinedList -destination 'platform=iOS Simulator,id=66CAE054-…' build — BUILD SUCCEEDED
• xcodebuild … -parallel-testing-enabled NO -skip-testing:InterlinedListTests/E2EReadOnlyTests test — 955 tests, 0 failures

E2E was skipped per the sandbox rules for this run; these changes are covered by the unit suite.

Notes / follow-ups

• Move-folder UI was left out. The issue lists it as optional and explicitly says not to block
  the rename on it. The API supports it (parentId: .root / .folder(id), both tested) and the
  400 cycle rejection is surfaced and rolled back through the same path as rename — there is just
  no picker wired into the UI yet, so a move isn't user-reachable.
• Backend ask: POST /api/documents/sync rejects folder updates. Until it grows an update
  branch for type: "folder", a rename can't be queued offline — it needs the network at the
  moment it's made, and the local mirror is what carries it across a relaunch.

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