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>

config/application.rb.dist

@@ -55,11 +55,8 @@ module FixinMe
     # Sender address of account registration-related messages
     Devise.mailer_sender = 'noreply@localhost'
 
-    # When set to true, new user registrations are automatically confirmed
-    # without requiring email verification, so accounts become active
-    # immediately upon sign-up.  Intended for installations where outgoing
-    # email is not configured, or for development / testing environments.
-    # Defaults to false (email confirmation is required).
-    # config.skip_email_confirmation = true
+    # Whether to skip e-mail confirmation for new registrations is configured
+    # through the web setup wizard and stored in the database (Setting model),
+    # so it does not need to be set here.
   end
 end