Live collaboration with Yjs over ActionCable

All editors now work on a server document at the same time instead of
taking turns behind an edit lock.

- The document's content is a sequence of Yjs updates (document_updates);
  DocumentChannel stores and relays them immediately, checks access on
  subscribe and again whenever links or memberships change.
- app/frontend/collab/: the store stays position-based; a binding maps it
  to a Yjs form with stable row/column ids, so a row inserted by one
  person doesn't misplace what another is typing. Remote changes are
  applied only after pending local ones went into Yjs.
- Per-person undo (Y.UndoManager), presence via awareness: the others'
  selections as coloured boxes with names, avatars in the toolbar.
- Existing JSON documents convert on first open, deterministically, so
  two people opening one at once end up with the same Yjs state.
- Editors compact the stored updates after 200 and keep a JSON snapshot
  current for export and duplicating. Edit lock and polling are gone.

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

app/controllers/api/base_controller.rb

@@ -17,12 +17,5 @@ module Api
 
     # a share link token, for visitors (with or without account) who got the link
     def share_token = request.headers["X-Share-Token"].presence
-
-    # identifies the lock holder: one browser tab
-    def client_id = request.headers["X-Client-Id"].to_s[0, 64].presence || "anon"
-
-    def lock_holder = "#{Current.user&.id || 'anon'}:#{client_id}"
-
-    def holder_name = Current.user&.name || "Jemand mit dem Link"
   end
 end