feat(21-01): add ErrorBoundary, Sonner toast provider, and 404 navigation

- Create ErrorBoundary class component with recovery UI (reload button)
- Create Sonner Toaster wrapper (bottom-right, richColors)
- Wire ErrorBoundary around Outlet in root route to catch render errors
- Add Toaster as sibling to AppLayout in root route
- Update notFoundComponent with Back to Dashboard link button

package-lock.json

@@ -5984,6 +5984,16 @@
         "url": "https://github.com/steveukx/git-js?sponsor=1"
       }
     },
+    "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": {
       "version": "0.6.1",
       "resolved": "https://registry.npmjs.org/source-map/-/source-map-0.6.1.tgz",
@@ -6725,6 +6735,7 @@
         "lucide-react": "^0.563.0",
         "react": "^19.0.0",
         "react-dom": "^19.0.0",
+        "sonner": "^2.0.7",
         "tailwind-merge": "^3.4.0"
       },
       "devDependencies": {