fix(ift): ObserverWeb mount at "" so CSS/JS paths are not //css-*

observer_dashboard("/") makes asset href protocol-relative (//css-HASH). Empty path keeps assets at /css-HASH.

docker/observer_web/dev.exs

@@ -12,8 +12,9 @@ defmodule WebDev.Router do
   scope "/" do
     pipe_through(:browser)
 
-    # Dedicated host observer.*.calentiq.com — mount at root (no /observer prefix)
+    # Dedicated host: mount at "" not "/" — library builds "#{prefix}/css-HASH",
-    observer_dashboard("/")
+    # and prefix="/" yields broken protocol-relative "//css-…".
+    observer_dashboard("")
   end
 end