feat(ui): group sidebar footer menus into distinct panels

The account and appearance menus sat in one container with no separation,
so expanding one made the other hard to find. Each collapsible menu now
lives in its own subtly lifted, bordered panel (.sidebar-panel) so the
expanded items stay visually contained and the menus never blend together.
Applied to the account, appearance, and sign-in menus.

web/static/input.css

@@ -212,6 +212,16 @@
     pointer-events: auto;
   }
 
+  /* Grouped panels for the collapsible footer menus (account, appearance).
+     Each menu lives in its own subtly lifted, bordered box so that expanded
+     submenus stay visually contained and one menu never blends into another. */
+  .sidebar-panel {
+    background-color: color-mix(in srgb, var(--text-primary) 4%, var(--bg-secondary));
+    border: 1px solid var(--border);
+    border-radius: 0.75rem;
+    padding: 0.375rem;
+  }
+
   /* Desktop: pin the sidebar, offset body content, drop the backdrop. */
   @media (min-width: 1024px) {
     body {