feat: add AllTube URL display on video load failure

Introduces a configuration property to specify an AllTube URL to be shown in error messages when video loading fails. Adds conditional logic to error handling to include a link to AllTube if the URL is configured and the request path meets criteria. This enhances user experience by providing an alternative viewing option.

src/invidious/config.cr

@@ -144,6 +144,9 @@ class Config
   # Playlist length limit
   property playlist_length_limit : Int32 = 500
 
+  # Display AllTube URL in error messages when loading a video fails
+  property alltube_url : String? = nil
+
   def disabled?(option)
     case disabled = CONFIG.disable_proxy
     when Bool

src/invidious/helpers/errors.cr

@@ -177,10 +177,20 @@ def error_redirect_helper(env : HTTP::Server::Context)
      request_path.starts_with?("/channel") || request_path.starts_with?("/playlist?list=PL")
     next_steps_text = translate(locale, "next_steps_error_message")
     refresh = translate(locale, "next_steps_error_message_refresh")
-    go_to_alltube = translate(locale, "next_steps_error_message_go_to_alltube")
     go_to_youtube = translate(locale, "next_steps_error_message_go_to_youtube")
     switch_instance = translate(locale, "Switch Invidious Instance")
 
+    if request_path.startswith?("/watch") && CONFIG.alltube_url
+      go_to_alltube = translate(locale, "next_steps_error_message_go_to_alltube")
+      alltube_part = <<-END_HTML
+        <li>
+          <a href="https://alltube.private.coffee#{env.request.resource}">#{go_to_alltube}</a>
+        </li>
+      END_HTML
+    else
+      alltube_part = ""
+    end
+
     return <<-END_HTML
       <p style="margin-bottom: 4px;">#{next_steps_text}</p>
       <ul>
@@ -190,9 +200,7 @@ def error_redirect_helper(env : HTTP::Server::Context)
         <li>
           <a href="/redirect?referer=#{env.get("current_page")}">#{switch_instance}</a>
         </li>
-        <li>
-          <a href="https://alltube.private.coffee#{env.request.resource}">#{go_to_alltube}</a>
-        </li>
+        #{alltube_part}
         <li>
           <a rel="noreferrer noopener" href="https://youtube.com#{env.request.resource}">#{go_to_youtube}</a>
         </li>