test: execute 30 smoke tests + fix P0 CSS break + BREAKS.md report

Layer 1 break detection results (21/30 pass, 63%):
- SaaS API: 5/5 pass (S3 skip no LLM key)
- Admin V2: 5/6 pass (A6 flaky auth guard)
- Desktop Chat: 3/6 pass (D1 no chat response in browser; D2/D3 skip non-Tauri)
- Desktop Feature: 6/6 pass
- Cross-System: 2/6 pass (4 blocked by login rate limit 429)

Bugs found:
- P0-01: Account lockout not enforced (locked_until set but not checked)
- P1-01: Refresh token still valid after logout
- P1-02: Desktop browser chat no response (stores not exposed)
- P1-03: Provider API requires display_name (undocumented)

Fixes applied:
- desktop/src/index.css: @import -> @plugin for Tailwind v4 compatibility
- Admin tests: correct credentials admin/admin123 from .env
- Cross tests: correct dashboard endpoint /stats/dashboard

admin-v2/package.json

@@ -26,6 +26,7 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.39.4",
+    "@playwright/test": "^1.59.1",
     "@tailwindcss/vite": "^4.2.2",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.2",