feat: Add onboarding tutorial and improved empty states

Onboarding Tutorial:
- 5-step walkthrough for new users
- Welcome, Scan, Taste, Sessions, Ready steps
- Skippable, stores completion in localStorage
- Beautiful full-screen overlay with animations

Empty States:
- SessionList: Visual empty state with icon and description
- BuddyList: Visual empty state with icon and description
- Reusable EmptyState component ready for more usage

Layout: Added OnboardingTutorial and CookieBanner

src/app/layout.tsx

@@ -11,6 +11,7 @@ import MainContentWrapper from "@/components/MainContentWrapper";
 import AuthListener from "@/components/AuthListener";
 import SyncHandler from "@/components/SyncHandler";
 import CookieBanner from "@/components/CookieBanner";
+import OnboardingTutorial from "@/components/OnboardingTutorial";
 
 const inter = Inter({ subsets: ["latin"], variable: '--font-inter' });
 
@@ -58,6 +59,7 @@ export default function RootLayout({
                             {children}
                         </MainContentWrapper>
                         <CookieBanner />
+                        <OnboardingTutorial />
                     </SessionProvider>
                 </I18nProvider>
             </body>