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/tailwind.config.js

@@ -22,7 +22,10 @@ export default {
           secondary: 'var(--fs-bronze)',
           destructive: 'var(--fs-oxblood)'
         },
-        accent: 'var(--fs-accent)',
+        accent: {
+          DEFAULT: 'var(--fs-accent)',
+          tint: 'color-mix(in srgb, var(--fs-accent) 12%, transparent)'
+        },
         warning: 'var(--fs-warning)',
         error: 'var(--fs-error)',
         info: 'var(--fs-info)'