fix(workspace): extract replaces the branch in the editor, with undo

The server only creates the new note and returns the link; the editor
swaps the branch for it as an ordinary own edit through the sync engine.
No more clash with unsent typing (conflict copies), the link shows at
once also with polling, and Strg+Z restores the branch. Undoing (toast or
Strg+Z) puts the new note into the trash via POST /extract/undo. The
sidebar gets the new note right away and opens its folder.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/server.go

@@ -136,6 +136,7 @@ func (s *Server) Router() http.Handler {
 		r.Post("/api/v1/documents/{id}/media/fetch", s.notInDemo(s.handleFetchMedia))
 		r.Get("/api/v1/media/{id}", s.handleGetMedia)
 		r.Post("/api/v1/documents/{id}/extract", s.handleExtractBranch)
+		r.Post("/api/v1/documents/{id}/extract/undo", s.handleUndoExtract)
 		r.Post("/api/v1/documents/{id}/merge", s.handleMergeDocuments)
 		r.Post("/api/v1/documents/{id}/merge/undo", s.handleUndoMerge)
 		r.Get("/api/v1/link-preview", s.notInDemo(s.handleLinkPreview))