Inject persistent top navbar (with printer switcher) into proxied Brother pages

app.rb

@@ -102,13 +102,18 @@ helpers do
 
     res = Net::HTTP.start(uri.host, uri.port) { |http| http.request(proxied) }
 
+    body = res.body || ''
+    if res['Content-Type']&.include?('text/html')
+      body = body.sub(/(<body[^>]*>)/i) { "#{Regexp.last_match(1)}\n#{erb(:_navbar, layout: false)}" }
+    end
+
     headers = {}
     res.each_header do |key, value|
       next if %w[transfer-encoding connection content-length].include?(key.downcase)
 
       headers[key] = value
     end
-    [res.code.to_i, headers, [res.body || '']]
+    [res.code.to_i, headers, [body]]
   end
 end