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>

Gemfile

@@ -0,0 +1,46 @@
+source "https://rubygems.org"
+
+# Bundle edge Rails instead: gem "rails", github: "rails/rails", branch: "main"
+gem "rails", "~> 8.1.3", ">= 8.1.3.1"
+# Use postgresql as the database for Active Record
+gem "pg", "~> 1.1"
+# Use the Puma web server [https://github.com/puma/puma]
+gem "puma", ">= 5.0"
+
+# Use Active Model has_secure_password [https://guides.rubyonrails.org/active_model_basics.html#securepassword]
+# gem "bcrypt", "~> 3.1.7"
+
+# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
+gem "tzinfo-data", platforms: %i[ windows jruby ]
+
+# Use the database-backed adapters for Rails.cache, Active Job, and Action Cable
+gem "solid_cache"
+gem "solid_queue"
+gem "solid_cable"
+
+# Reduces boot times through caching; required in config/boot.rb
+gem "bootsnap", require: false
+
+group :development, :test do
+  # See https://guides.rubyonrails.org/debugging_rails_applications.html#debugging-with-the-debug-gem
+  gem "debug", platforms: %i[ mri windows ], require: "debug/prelude"
+
+  # Audits gems for known security defects (use config/bundler-audit.yml to ignore issues)
+  gem "bundler-audit", require: false
+
+  # Static analysis for security vulnerabilities [https://brakemanscanner.org/]
+  gem "brakeman", require: false
+end
+
+group :development do
+  # Use console on exceptions pages [https://github.com/rails/web-console]
+  gem "web-console"
+end
+
+# Vite as the frontend build, integrated into Rails [https://vite-ruby.netlify.app]
+gem "vite_rails"
+
+group :development do
+  # Mails open in the browser at /letter_opener instead of being sent
+  gem "letter_opener_web"
+end