FIX: Add theme-color <meta> tag when a dark scheme is selected (#18747)

Meta topic: https://meta.discourse.org/t/meta-theme-color-is-not-respecting-current-color-scheme/239815/7?u=osama.

This commit renders an additional `theme-color` `<meta>` tag for the dark scheme if the current user/request has a scheme selected for dark mode. We currently only render one `theme-color` tag which is always based on the user's selected scheme for light mode, but if the user also selects a scheme for dark mode and uses a device that's configured to use/prefer dark mode, the Discourse UI will be in dark mode, but any parts of the browser/OS UI that's colored based on the `theme-color` tag, would use a color from the user's selected light scheme and look inconsistent with the Discourse UI because the `theme-color` tag is based on the user's selected light scheme.

The additional `theme-color` tag has `media="(prefers-color-scheme: dark)"` and is based on the user's selected dark scheme which means any browser UI that's colored based on `theme-color` tags should be able to pick the right tag based on the user's preference for light/dark mode.

app/helpers/application_helper.rb

@@ -625,6 +625,18 @@ module ApplicationHelper
     result.html_safe
   end
 
+  def discourse_theme_color_meta_tags
+    result = +<<~HTML
+      <meta name="theme-color" media="all" content="##{ColorScheme.hex_for_name('header_background', scheme_id)}">
+    HTML
+    if dark_scheme_id != -1
+      result << <<~HTML
+        <meta name="theme-color" media="(prefers-color-scheme: dark)" content="##{ColorScheme.hex_for_name('header_background', dark_scheme_id)}">
+      HTML
+    end
+    result.html_safe
+  end
+
   def dark_color_scheme?
     return false if scheme_id.blank?
     ColorScheme.find_by_id(scheme_id)&.is_dark?