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

package-lock.json

@@ -41,6 +41,7 @@
                 "react-drawio": "^1.0.3",
                 "react-icons": "^5.5.0",
                 "remark-gfm": "^4.0.1",
+                "sonner": "^2.0.7",
                 "tailwind-merge": "^3.0.2",
                 "tailwindcss-animate": "^1.0.7",
                 "zod": "^4.1.12"
@@ -10322,6 +10323,16 @@
                 "url": "https://github.com/sponsors/ljharb"
             }
         },
+        "node_modules/sonner": {
+            "version": "2.0.7",
+            "resolved": "https://registry.npmjs.org/sonner/-/sonner-2.0.7.tgz",
+            "integrity": "sha512-W6ZN4p58k8aDKA4XPcx2hpIQXBRAgyiWVkYhT7CvK6D3iAu7xjvVyhQHg2/iaKJZ1XVJ4r7XuwGL+WGEK37i9w==",
+            "license": "MIT",
+            "peerDependencies": {
+                "react": "^18.0.0 || ^19.0.0 || ^19.0.0-rc",
+                "react-dom": "^18.0.0 || ^19.0.0 || ^19.0.0-rc"
+            }
+        },
         "node_modules/source-map-js": {
             "version": "1.2.1",
             "resolved": "https://registry.npmjs.org/source-map-js/-/source-map-js-1.2.1.tgz",