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/postcss.config.js

@@ -1,6 +1,5 @@
 export default {
   plugins: {
-    tailwindcss: {},
-    autoprefixer: {},
+    '@tailwindcss/postcss': {},
   },
 }