chore(08-08): PERF-01 packages + Vite 6 upgrade + pin requirements.txt

- Upgraded vite 5.4.21 → 6.4.3 (fixes CVE-2026-39363, CVE-2026-39364)
- Upgraded @vitejs/plugin-vue 5.2.4 → 6.0.7 (Vite 6 peer dep)
- Installed @vueuse/core@14.3.0, @vueuse/integrations@14.3.0,
  sortablejs@1.15.7, @tailwindcss/forms@0.5.11,
  rollup-plugin-visualizer@7.0.1, @types/sortablejs@1.15.9
- Wired @tailwindcss/forms plugin in tailwind.config.js (VISUAL-02 ready)
- Pinned backend/requirements.txt to exact == versions per D-17 (32 packages)
- Frontend: 136/136 tests pass on Vite 6; 0 npm audit vulnerabilities
- Backend: 408 passed, 4 skipped, 7 xfailed (baseline 405+1)
- Phase 8 complete (8/8 plans)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/tailwind.config.js

@@ -1,8 +1,9 @@
 /** @type {import('tailwindcss').Config} */
+import forms from '@tailwindcss/forms'
 export default {
   content: ['./index.html', './src/**/*.{vue,js}'],
   theme: {
     extend: {},
   },
-  plugins: [],
+  plugins: [forms],
 }