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>

config/initializers/content_security_policy.rb

@@ -0,0 +1,38 @@
+# Be sure to restart your server when you modify this file.
+
+# Define an application-wide content security policy.
+# See the Securing Rails Applications Guide for more information:
+# https://guides.rubyonrails.org/security.html#content-security-policy-header
+
+# Rails.application.configure do
+#   config.content_security_policy do |policy|
+#     policy.default_src :self, :https
+#     policy.font_src    :self, :https, :data
+#     policy.img_src     :self, :https, :data
+#     policy.object_src  :none
+#     policy.script_src  :self, :https
+    # Allow @vite/client to hot reload javascript changes in development
+#    policy.script_src *policy.script_src, :unsafe_eval, "http://#{ ViteRuby.config.host_with_port }" if Rails.env.development?
+
+    # You may need to enable this in production as well depending on your setup.
+#    policy.script_src *policy.script_src, :blob if Rails.env.test?
+
+#     policy.style_src   :self, :https
+    # Allow @vite/client to hot reload style changes in development
+#    policy.style_src *policy.style_src, :unsafe_inline if Rails.env.development?
+
+#     # Specify URI for violation reports
+#     # policy.report_uri "/csp-violation-report-endpoint"
+#   end
+#
+#   # Generate session nonces for permitted importmap, inline scripts, and inline styles.
+#   config.content_security_policy_nonce_generator = ->(request) { request.session.id.to_s }
+#   config.content_security_policy_nonce_directives = %w(script-src style-src)
+#
+#   # Automatically add `nonce` to `javascript_tag`, `javascript_include_tag`, and `stylesheet_link_tag`
+#   # if the corresponding directives are specified in `content_security_policy_nonce_directives`.
+#   # config.content_security_policy_nonce_auto = true
+#
+#   # Report violations without enforcing the policy.
+#   # config.content_security_policy_report_only = true
+# end