chore: remove dead code and unused dependencies

Deleted:
- libs/utils/src/react-router/ — old file-based routing system (replaced by TanStack Router)
- next-themes dependency (unused since Astro migration)
- react-router-dom from vite external config

Cleaned:
- Removed react-router barrel export from utils/index.ts
- Kept landing data files (still used by Astro components)
- Kept QueryProvider and ModalLoginProvider (still used by apps)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package-lock.json

@@ -32,7 +32,6 @@
         "graphql": "^16.13.2",
         "html2canvas": "^1.4.1",
         "js-cookie": "^3.0.5",
-        "next-themes": "^0.4.6",
         "openapi-fetch": "^0.17.0",
         "openapi-react-query": "^0.5.4",
         "picomatch": "^4.0.4",
@@ -20230,16 +20229,6 @@
         "node": ">= 10"
       }
     },
-    "node_modules/next-themes": {
-      "version": "0.4.6",
-      "resolved": "https://registry.npmjs.org/next-themes/-/next-themes-0.4.6.tgz",
-      "integrity": "sha512-pZvgD5L0IEvX5/9GWyHMf3m8BKiVQwsCMHfoFosXtXBMnaS0ZnIJ9ST4b4NqLVKDEm8QBxoNNGNaBv2JNF6XNA==",
-      "license": "MIT",
-      "peerDependencies": {
-        "react": "^16.8 || ^17 || ^18 || ^19 || ^19.0.0-rc",
-        "react-dom": "^16.8 || ^17 || ^18 || ^19 || ^19.0.0-rc"
-      }
-    },
     "node_modules/nlcst-to-string": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/nlcst-to-string/-/nlcst-to-string-4.0.0.tgz",