fix(web): give user-menu dropdown z-50 so clicks don't fall through

The Shell's username dropdown had no z-index, so the layout's later
grid siblings (left nav, main content) painted over it. Visible
symptom: clicking Admin actually clicked the Home link underneath,
producing a quick flash followed by a redirect to /. Adding z-50 to
the popup container makes clicks land where they look like they should.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/components/Shell.svelte

@@ -52,7 +52,7 @@
       </button>
       {#if menuOpen}
         <div
-          class="absolute right-0 mt-1 w-40 rounded border border-border bg-surface shadow"
+          class="absolute right-0 z-50 mt-1 w-40 rounded border border-border bg-surface shadow"
           onclick={(e) => e.stopPropagation()}
           onkeydown={(e) => e.stopPropagation()}
           role="menu"