UX: add color-scheme meta tag to _head (#30245)

Adds the `color-scheme` meta tag to the `_head` partial and removes it from the finish installation template to prevent it from being added twice.

app/helpers/application_helper.rb

@@ -678,6 +678,20 @@ module ApplicationHelper
     result.html_safe
   end
 
+  def discourse_color_scheme_meta_tag
+    scheme =
+      if dark_scheme_id == -1
+        # no automatic client-side switching
+        dark_color_scheme? ? "dark" : "light"
+      else
+        # auto-switched based on browser setting
+        "light dark"
+      end
+    <<~HTML.html_safe
+        <meta name="color-scheme" content="#{scheme}">
+      HTML
+  end
+
   def dark_color_scheme?
     return false if scheme_id.blank?
     ColorScheme.find_by_id(scheme_id)&.is_dark?