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/views/setup/new.html.erb

@@ -0,0 +1,38 @@
+<%= form_with url: setup_path, method: :post, class: "main-area" do |f| %>
+  <fieldset>
+    <legend><%= t(".admin_account") %></legend>
+
+    <label for="admin_email"><%= t(".admin_email") %></label>
+    <%= email_field_tag :admin_email, params[:admin_email],
+          id: "admin_email", required: true, size: 30, autofocus: true,
+          autocomplete: "email" %>
+
+    <label for="admin_password"><%= t(".admin_password") %></label>
+    <%= password_field_tag :admin_password, nil,
+          id: "admin_password", required: true, size: 30,
+          autocomplete: "new-password" %>
+
+    <label for="admin_password_confirmation"><%= t(".admin_password_confirmation") %></label>
+    <%= password_field_tag :admin_password_confirmation, nil,
+          id: "admin_password_confirmation", required: true, size: 30,
+          autocomplete: "off" %>
+  </fieldset>
+
+  <fieldset>
+    <legend><%= t(".options") %></legend>
+
+    <label for="skip_email_confirmation">
+      <%= check_box_tag :skip_email_confirmation, "1",
+            params[:skip_email_confirmation] == "1",
+            id: "skip_email_confirmation" %>
+      <%= t(".skip_email_confirmation") %>
+    </label>
+
+    <label for="seed_units">
+      <%= check_box_tag :seed_units, "1", true, id: "seed_units" %>
+      <%= t(".seed_units") %>
+    </label>
+  </fieldset>
+
+  <%= submit_tag t(".submit") %>
+<% end %>