feat: add image upload validation with 2MB limit and max 5 files (#101)

- Add 2MB file size limit with client and server-side validation
- Add max 5 files limit per upload
- Add sonner toast library for better error notifications
- Create ErrorToast component with keyboard accessibility
- Batch multiple validation errors into single toast
- Validate file size in all upload methods (input, paste, drag-drop)
- Add server-side validation in /api/chat endpoint

app/layout.tsx

@@ -96,7 +96,6 @@ export default function RootLayout({
                 className={`${plusJakarta.variable} ${jetbrainsMono.variable} antialiased`}
             >
                 <DiagramProvider>{children}</DiagramProvider>
-
                 <Analytics />
             </body>
             {process.env.NEXT_PUBLIC_GA_ID && (