feat(routing): switch from hash routing to HTML5 History API

Remove all window.location.hash assignments and hashchange events.
Navigate with history.pushState; listen on popstate. Fix base href to
'/' so assets resolve correctly on deep paths. Update magic link URL.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/index.html

@@ -16,7 +16,7 @@
     })();
   </script>
 
-  <base href="./">
+  <base href="/">
 
 
   <link rel="icon" href="data:;base64,iVBORw0KGgo=">