Serve custom styles from a dedicated text/css route

Admin-authored custom styles were rendered inline into every page via
custom_styles_tag, marked html_safe inside a <style> element. A stored
payload containing </style><script>…</script> broke out of the style
context and executed as HTML — a stored XSS reaching everyone who
loads the app.

Deliver the styles as an external stylesheet instead, following the
account logo pattern: a new Accounts::CustomStyles#show renders the
raw CSS with a text/css content type (ETag on the account, 5-minute
public cache with stale-while-revalidate, fresh_custom_styles cache
buster on account updates), and the layout links it with
<link rel="stylesheet" data-turbo-track="reload">. In a CSS
document, markup is inert text — the HTML-context breakout is closed
by construction, and admins keep full custom-CSS capability.

test/controllers/accounts/custom_styles_controller_test.rb

@@ -5,6 +5,34 @@ class Accounts::CustomStylesControllerTest < ActionDispatch::IntegrationTest
     sign_in :david
   end
 
+  test "show serves custom styles as plain CSS" do
+    accounts(:signal).update! custom_styles: ":root { --color-text: red; }"
+
+    get account_custom_styles_url
+    assert_response :ok
+    assert_equal "text/css", @response.media_type
+    assert_equal ":root { --color-text: red; }", @response.body
+  end
+
+  test "show is accessible without authentication" do
+    accounts(:signal).update! custom_styles: ":root { --color-text: red; }"
+    reset!
+
+    get account_custom_styles_url
+    assert_response :ok
+    assert_equal "text/css", @response.media_type
+  end
+
+  test "show serves markup verbatim as inert CSS text, never HTML" do
+    payload = "</style><script>alert(1)</script>"
+    accounts(:signal).update! custom_styles: payload
+
+    get account_custom_styles_url
+    assert_response :ok
+    assert_equal "text/css", @response.media_type
+    assert_equal payload, @response.body
+  end
+
   test "edit" do
     get edit_account_custom_styles_url
     assert_response :ok