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.

ui/tsconfig.json

@@ -16,9 +16,8 @@
     "noUnusedParameters": true,
     "noFallthroughCasesInSwitch": true,
     "noUncheckedSideEffectImports": true,
-    "baseUrl": ".",
     "paths": {
-      "@/*": ["src/*"]
+      "@/*": ["./src/*"]
     }
   },
   "include": ["src/**/*.ts", "src/**/*.tsx", "src/**/*.vue"]