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/locales/en.yml

@@ -163,6 +163,23 @@ en:
           <br><em>leave blank to keep unchanged</em>
           %{password_length_hint_html}
   actions: Actions
+  setup:
+    new:
+      admin_account: Admin account
+      admin_email: 'E-mail:'
+      admin_password: 'Password:'
+      admin_password_confirmation: 'Retype password:'
+      options: Options
+      skip_email_confirmation: Skip e-mail confirmation for new registrations
+      seed_units: Seed built-in default units
+      submit: Set up
+    create:
+      email_blank: E-mail cannot be blank.
+      password_blank: Password cannot be blank.
+      password_mismatch: Passwords do not match.
+      success: >
+        Installation complete. You can now sign in with the admin account you
+        just created.
   add: Add
   apply: Apply
   back: Back