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/locales/en.yml

@@ -0,0 +1,31 @@
+# Files in the config/locales directory are used for internationalization and
+# are automatically loaded by Rails. If you want to use locales other than
+# English, add the necessary files in this directory.
+#
+# To use the locales, use `I18n.t`:
+#
+#     I18n.t "hello"
+#
+# In views, this is aliased to just `t`:
+#
+#     <%= t("hello") %>
+#
+# To use a different locale, set it with `I18n.locale`:
+#
+#     I18n.locale = :es
+#
+# This would use the information in config/locales/es.yml.
+#
+# To learn more about the API, please read the Rails Internationalization guide
+# at https://guides.rubyonrails.org/i18n.html.
+#
+# Be aware that YAML interprets the following case-insensitive strings as
+# booleans: `true`, `false`, `on`, `off`, `yes`, `no`. Therefore, these strings
+# must be quoted to be interpreted as strings. For example:
+#
+#     en:
+#       "yes": yup
+#       enabled: "ON"
+
+en:
+  hello: "Hello world"