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

components/error-toast.tsx

@@ -0,0 +1,39 @@
+"use client";
+
+import React from "react";
+
+interface ErrorToastProps {
+    message: React.ReactNode;
+    onDismiss: () => void;
+}
+
+export function ErrorToast({ message, onDismiss }: ErrorToastProps) {
+    const handleKeyDown = (e: React.KeyboardEvent) => {
+        if (e.key === "Enter" || e.key === " " || e.key === "Escape") {
+            e.preventDefault();
+            onDismiss();
+        }
+    };
+
+    return (
+        <div
+            role="alert"
+            aria-live="polite"
+            tabIndex={0}
+            onClick={onDismiss}
+            onKeyDown={handleKeyDown}
+            className="flex items-center gap-3 bg-card border border-border/50 px-4 py-3 rounded-xl shadow-sm cursor-pointer hover:bg-muted/50 focus:outline-none focus:ring-2 focus:ring-primary/50 transition-colors"
+        >
+            <div className="flex items-center justify-center w-8 h-8 rounded-full bg-destructive/10 flex-shrink-0">
+                <svg className="w-4 h-4 text-destructive" viewBox="0 0 20 20" fill="currentColor" aria-hidden="true">
+                    <path
+                        fillRule="evenodd"
+                        d="M4.293 4.293a1 1 0 011.414 0L10 8.586l4.293-4.293a1 1 0 111.414 1.414L11.414 10l4.293 4.293a1 1 0 01-1.414 1.414L10 11.414l-4.293 4.293a1 1 0 01-1.414-1.414L8.586 10 4.293 5.707a1 1 0 010-1.414z"
+                        clipRule="evenodd"
+                    />
+                </svg>
+            </div>
+            <span className="text-sm text-foreground">{message}</span>
+        </div>
+    );
+}