Add Rails backend: accounts via magic link, server documents, sharing

The app becomes a Rails 8 application; the Vue frontend moves to
app/frontend and is served through vite_rails.

- Without an account documents stay in the browser (IndexedDB). Sign-in
  is by magic link or the 6-digit code from the same mail; afterwards
  local documents can be uploaded into the account.
- Server documents (/d/<id>) with versioned saves and a single-editor
  lock per browser tab; others see them read-only and follow by polling.
- Sharing via link (works without an account) and via e-mail invitation,
  roles viewer/editor; the owner manages links and people.
- Store gains a read-only mode that blocks all content changes.
- Solid Queue, Cache and Cable share the one PostgreSQL database.

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

app/controllers/api/joins_controller.rb

@@ -0,0 +1,17 @@
+module Api
+  # POST /api/documents/:document_id/join { token }
+  # Redeems a share link. Signed in: the document is added to the account
+  # (a membership). Not signed in: only answers what the token grants -- the
+  # browser keeps the token and sends it along (X-Share-Token).
+  class JoinsController < BaseController
+    def create
+      doc = Document.find(params[:document_id])
+      link = doc.share_links.active.find_by(token: params.require(:token))
+      raise ActiveRecord::RecordNotFound unless link
+      if Current.user
+        DocumentMembership.grant!(document: doc, user: Current.user, role: link.role, source: :link, share_link: link)
+      end
+      render json: { document: doc.meta_json(role: doc.role_for(user: Current.user, token: link.token)) }
+    end
+  end
+end