Add OTP login flow and multi-cafe role switching

Introduce an OTP input box on login/register, surface user roles and a
cafe chooser, add a dashboard switch button in the POS screen, and
register OTP validators explicitly to survive Docker layer caching.

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

web/dashboard/src/components/pos/pos-screen.tsx

@@ -7,6 +7,7 @@ import { useTranslations, useLocale } from "next-intl";
 import {
   ChevronLeft,
   ChevronRight,
+  LayoutDashboard,
   Minus,
   Package,
   Plus,
@@ -899,6 +900,19 @@ export function PosScreen() {
         >
           {t("modePay")}
         </Button>
+
+        <div className="flex-1" />
+
+        {/* Dashboard shortcut — only visible to Owner / Manager */}
+        {isManager && (
+          <a
+            href="/"
+            className="flex items-center gap-1.5 rounded-lg px-2.5 py-1.5 text-xs text-muted-foreground transition-colors hover:bg-accent hover:text-foreground"
+          >
+            <LayoutDashboard className="size-4" />
+            <span className="hidden sm:inline">{cafeName}</span>
+          </a>
+        )}
       </div>
 
       {/* ── Pay mode ──────────────────────────────────────────────────────── */}