Upgrade to ESLint 9 for Next.js 16 compatibility

Next.js 16 requires ESLint 9 for proper flat config support. This commit:
- Upgrades eslint from ^8 to ^9
- Migrates from .eslintrc.json to eslint.config.mjs (flat config)
- Updates lint script to use eslint directly (next lint removed in Next.js 16)
- Adds tsconfig.json includes for Next.js 16 dev types
- Fixes layout.tsx to use proper theme CSS link tags

Signed-off-by: Charles de Dreuille <charles.dedreuille@gmail.com>

docs-ui/tsconfig.json

@@ -1,6 +1,10 @@
 {
   "compilerOptions": {
-    "lib": ["dom", "dom.iterable", "esnext"],
+    "lib": [
+      "dom",
+      "dom.iterable",
+      "esnext"
+    ],
     "allowJs": true,
     "skipLibCheck": true,
     "strict": false,
@@ -19,7 +23,9 @@
       }
     ],
     "paths": {
-      "@/*": ["./src/*"]
+      "@/*": [
+        "./src/*"
+      ]
     },
     "target": "ES2017"
   },
@@ -30,7 +36,10 @@
     ".next/types/**/*.ts",
     "dist/types/**/*.ts",
     "src/types/**/*.d.ts",
-    "dist/dev/types/**/*.ts"
+    "dist/dev/types/**/*.ts",
+    ".next/dev/types/**/*.ts"
   ],
-  "exclude": ["node_modules"]
+  "exclude": [
+    "node_modules"
+  ]
 }