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>

db/seeds.rb

@@ -3,6 +3,17 @@
 #   bin/rails db:seed
 # command (or created alongside the database with db:setup).
 # Seeding process should be idempotent.
+#
+# Admin account setup
+# -------------------
+# The preferred way to create the first admin account is through the web setup
+# wizard, which is shown automatically on the first visit when no admin exists.
+# The wizard also lets you configure runtime options (e.g. skip e-mail
+# confirmation) and seed the default units without using the command line.
+#
+# The block below provides an alternative CLI path for headless / automated
+# deployments.  It is skipped when an admin account already exists (e.g. after
+# the web wizard has run).
 
 User.transaction do
   break if User.find_by status: :admin