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/setup_controller.rb

@@ -0,0 +1,59 @@
+# Handles the one-time web-based installation wizard.
+#
+# The wizard is only accessible when no admin account exists yet.  Once an
+# admin has been created the controller redirects every request to the root
+# path, so it can never be used to overwrite an existing installation.
+class SetupController < ActionController::Base
+  # Use the full application layout (header, flash, etc.) so the page looks
+  # consistent with the rest of the site.
+  layout "application"
+
+  before_action :redirect_if_installed
+
+  def new
+  end
+
+  def create
+    email    = params[:admin_email].to_s.strip
+    password = params[:admin_password].to_s
+    confirm  = params[:admin_password_confirmation].to_s
+
+    errors = []
+    errors << t(".email_blank")    if email.blank?
+    errors << t(".password_blank") if password.blank?
+    errors << t(".password_mismatch") if password != confirm
+
+    if errors.any?
+      flash.now[:alert] = errors.join("  ")
+      return render :new, status: :unprocessable_entity
+    end
+
+    user = User.new(email: email, password: password, status: :admin)
+    user.skip_confirmation!
+
+    unless user.save
+      flash.now[:alert] = user.errors.full_messages.join("  ")
+      return render :new, status: :unprocessable_entity
+    end
+
+    # Persist runtime settings chosen during setup.
+    Setting.set("skip_email_confirmation",
+                params[:skip_email_confirmation] == "1")
+
+    # Optionally seed the built-in default units.
+    if params[:seed_units] == "1"
+      load Rails.root.join("db/seeds/units.rb")
+    end
+
+    redirect_to new_user_session_path, notice: t(".success")
+  end
+
+  private
+
+  def redirect_if_installed
+    redirect_to root_path if User.exists?(status: :admin)
+  rescue ActiveRecord::StatementInvalid
+    # Tables are not yet migrated — stay on the setup page so the user sees a
+    # meaningful error rather than a crash.
+  end
+end