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/models/setting.rb

@@ -0,0 +1,20 @@
+# Key-value store for runtime application settings that are configured through
+# the web setup wizard (or updated by an administrator) rather than hard-coded
+# in application.rb.
+#
+# Known keys:
+#   skip_email_confirmation  – "true"/"false", mirrors the homonymous option
+#                              that was previously in application.rb.
+class Setting < ApplicationRecord
+  validates :key, presence: true, uniqueness: true
+
+  # Return the string value stored for +key+, or +default+ when absent.
+  def self.get(key, default: nil)
+    find_by(key: key)&.value || default
+  end
+
+  # Persist +value+ for +key+, creating the record if it does not yet exist.
+  def self.set(key, value)
+    find_or_initialize_by(key: key).update!(value: value.to_s)
+  end
+end