Update Node to 26 and dependencies to their latest versions

- Node 20 is end-of-life; use the current LTS in the Docker image
- Major updates: @sanity/client 8, @sanity/image-url 2,
  @portabletext/react 8, eslint-config-next 16
- Unpin React and update to 19.3
- ESLint stays on 9 and TypeScript on 6.0, since eslint-config-next's
  plugins and typescript-eslint don't support newer majors yet
- Use the native flat config from eslint-config-next 16
- Use useSyncExternalStore for WebGL detection to satisfy the new
  react-hooks lint rule

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

package.json

@@ -12,28 +12,27 @@
     "format": "prettier --write \"**/*.{js,jsx,ts,tsx,json,md}\""
   },
   "dependencies": {
-    "@portabletext/react": "^3.2.1",
-    "@sanity/client": "^7.8.1",
-    "@sanity/image-url": "^1.1.0",
+    "@portabletext/react": "^8.0.1",
+    "@sanity/client": "^8.14.1",
+    "@sanity/image-url": "^2.1.1",
     "@sanity/webhook": "^4.0.4",
-    "country-flag-icons": "^1.5.19",
+    "country-flag-icons": "^1.6.20",
     "install": "^0.13.0",
-    "next": "^16.3.3",
-    "next-intl": "^4.3.4",
-    "react": "19.1.0",
-    "react-dom": "19.1.0",
-    "react-icons": "^5.5.0",
+    "next": "^16.4.3",
+    "next-intl": "^4.14.9",
+    "react": "^19.3.1",
+    "react-dom": "^19.3.1",
+    "react-icons": "^5.7.0",
     "react-photo-sphere-viewer": "^6.2.3"
   },
   "devDependencies": {
-    "@eslint/eslintrc": "^3",
-    "@types/node": "^20",
-    "@types/react": "^19",
-    "@types/react-dom": "^19",
-    "eslint": "^9",
-    "eslint-config-next": "15.4.4",
-    "prettier": "^3.6.2",
-    "typescript": "^5"
+    "@types/node": "^26.6.6",
+    "@types/react": "^19.3.0",
+    "@types/react-dom": "^19.3.0",
+    "eslint": "^9.39.5",
+    "eslint-config-next": "^16.4.3",
+    "prettier": "^3.9.10",
+    "typescript": "~6.0.3"
   },
   "repository": {
     "type": "git",