chore: upgrade ui and docs frontend dependencies to latest

ui: Vite 8, vue-tsc 3, TypeScript 6, @vitejs/plugin-vue 6, vis-network 10,
vis-data 8, FontAwesome 7, @types/node 25. docs: Vite 8, plugin-vue 6.
Both migrated to Tailwind CSS v4.

- Tailwind v3 -> v4: @import "tailwindcss" with @theme/@config, switch to
  @tailwindcss/postcss, drop autoprefixer (bundled via Lightning CSS).
- docs: move theme into @theme, delete tailwind.config.js, remove stale
  pnpm-lock.yaml (npm/package-lock.json is authoritative).
- ui: TypeScript 6 removes deprecated tsconfig baseUrl; no code changes
  needed for vis-network 10 / FontAwesome 7.
- Bump CI Node 20 -> 22 (Vite 8 requires Node ^20.19 || >=22.12).

0 npm audit vulnerabilities; ui type-check and both production builds pass.

docs/package.json

@@ -8,13 +8,13 @@
     "preview": "vite preview"
   },
   "dependencies": {
-    "vue": "^3.5.25"
+    "vue": "^3.5.34"
   },
   "devDependencies": {
-    "@vitejs/plugin-vue": "^6.0.3",
-    "autoprefixer": "^10.4.23",
-    "postcss": "^8.5.6",
-    "tailwindcss": "^3.4.19",
-    "vite": "^7.3.0"
+    "@tailwindcss/postcss": "^4.3.0",
+    "@vitejs/plugin-vue": "^6.0.7",
+    "postcss": "^8.5.15",
+    "tailwindcss": "^4.3.0",
+    "vite": "^8.0.14"
   }
 }