fix(web): promote bg-accent-tint to a real Tailwind utility

Replaces the :global scoped-style workaround in AdminSidebar with a proper
accent.tint color tier in tailwind.config.js. Tailwind generates bg-accent-tint
deterministically; the global class no longer leaks from a single component.
Future consumers (e.g. /admin/requests tab counts) get the same utility.

web/src/lib/components/AdminSidebar.svelte

@@ -57,12 +57,3 @@
     </ul>
   </nav>
 </aside>
-
-<style>
-  /* "12% accent-tinted bg" — the design-system rule for active nav state.
-     :global is required because the class is composed inside a templated
-     class string the Svelte scoped-CSS pass can't see. */
-  :global(.bg-accent-tint) {
-    background: color-mix(in srgb, var(--fs-accent) 12%, transparent);
-  }
-</style>