feat(web): add light/dark theme toggle to public nav

Public layout only had the language switcher next to the auth block —
users reading the public site had no way to toggle theme, while the
dashboard layout has always had one. Mirror the dashboard's toggle:
Moon icon when the app is in light mode, Sun icon when in dark mode,
aria-label pulled from the `dashboard.darkMode`/`lightMode` strings
that are already translated.

Sits between LanguageSwitcher and the user/auth block so it's visible
on both desktop and mobile headers without adding to the hamburger
menu.

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

apps/web/app/[locale]/(public)/layout.tsx

@@ -1,11 +1,12 @@
 'use client';
 
-import { LogOut, Menu, User as UserIcon, X } from 'lucide-react';
+import { LogOut, Menu, Moon, Sun, User as UserIcon, X } from 'lucide-react';
 import { usePathname } from 'next/navigation';
 import { useTranslations } from 'next-intl';
 import { useState } from 'react';
 import { CompareFloatingBar } from '@/components/comparison/compare-floating-bar';
 import { NotificationBell } from '@/components/notifications/notification-bell';
+import { useTheme } from '@/components/providers/theme-provider';
 import { Badge } from '@/components/ui/badge';
 import { Button } from '@/components/ui/button';
 import { LanguageSwitcher } from '@/components/ui/language-switcher';
@@ -33,6 +34,7 @@ export default function PublicLayout({ children }: { children: React.ReactNode }
   const pathname = usePathname();
   const router = useRouter();
   const { user, logout } = useAuthStore();
+  const { theme, toggleTheme } = useTheme();
   const t = useTranslations();
   const [mobileMenuOpen, setMobileMenuOpen] = useState(false);
 
@@ -109,6 +111,19 @@ export default function PublicLayout({ children }: { children: React.ReactNode }
 
           <div className="ml-auto flex min-w-0 items-center gap-1 sm:gap-2">
             <LanguageSwitcher />
+            <Button
+              variant="ghost"
+              size="sm"
+              onClick={toggleTheme}
+              aria-label={theme === 'light' ? t('dashboard.darkMode') : t('dashboard.lightMode')}
+              className="h-9 w-9 shrink-0 p-0"
+            >
+              {theme === 'light' ? (
+                <Moon className="h-4 w-4" aria-hidden="true" />
+              ) : (
+                <Sun className="h-4 w-4" aria-hidden="true" />
+              )}
+            </Button>
             {user ? (
               <>
                 {/* Bell only on sm+ to avoid overcrowding mobile header */}