Add web-based installation wizard

Replace the CLI-only setup (db:seed + manual application.rb edits)
with a web wizard shown automatically on first visit when no admin
account exists yet.

SetupController (GET/POST /setup) collects the admin e-mail and
password, a "skip e-mail confirmation" toggle, and an option to
seed the built-in default units.  Once submitted it creates the
admin User, persists the chosen options as Setting records, and
redirects to the sign-in page.

ApplicationController gains a redirect_to_setup_if_needed
before_action that catches every request (including Devise routes)
when no admin exists, so a fresh installation always lands on the
wizard rather than an empty sign-in form.

A new Setting model provides a lightweight key-value store for
runtime options that were previously hard-coded in application.rb
(e.g. skip_email_confirmation).  RegistrationsController now reads
that flag from the database instead of from the application config.

Seeds.rb is kept for headless / automated deployments and skips
admin creation when an admin already exists (idempotent), with a
comment pointing to the web wizard as the preferred path.

Also extends the SQLite nil-limit fix (|| Float::INFINITY) to the
Quantity model, which suffered the same ArgumentError as Unit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/controllers/application_controller.rb

@@ -9,6 +9,7 @@ class ApplicationController < ActionController::Base
   helper_method :current_user_disguised?
   helper_method :current_tab
 
+  before_action :redirect_to_setup_if_needed
   before_action :authenticate_user!
 
   class AccessForbidden < StandardError; end
@@ -43,6 +44,16 @@ class ApplicationController < ActionController::Base
 
   private
 
+  # Redirect to the web setup wizard when the application has not yet been
+  # initialised (i.e. no admin account exists in the database).
+  def redirect_to_setup_if_needed
+    return if User.exists?(status: :admin)
+    redirect_to new_setup_path
+  rescue ActiveRecord::StatementInvalid
+    # Tables may not exist yet (migrations not run). Fall through and let the
+    # normal request handling surface a meaningful error.
+  end
+
   def render_no_content(record)
     helpers.render_errors(record)
     render html: nil, layout: true