feat(security): HttpOnly cookies + CSP headers + CSRF double-submit protection

- Migrate auth tokens from localStorage to HttpOnly SameSite=Lax cookies
- Add cookie_auth.rs: helpers for setting/clearing auth + CSRF cookies
- Update auth middleware: 3-method auth (Bearer → Basic → Cookie)
- Add 5 security headers: CSP, X-Content-Type-Options, X-Frame-Options,
  Referrer-Policy, Permissions-Policy
- Implement CSRF double-submit cookie pattern (csrf.rs middleware)
- Set CSRF cookie on login/refresh/oidc-exchange, clear on logout
- CookieAuthenticated marker skips CSRF for Bearer/Basic clients
- Frontend: strip all localStorage token refs from 14 JS files
- Frontend: csrf.js utility + all 52 mutating fetch/XHR calls protected
- 121 tests passing, 0 warnings

static/admin.html

@@ -6,6 +6,7 @@
 <title>OxiCloud — Admin Panel</title>
 <script>if(localStorage.getItem('oxicloud_theme')==='dark')document.documentElement.setAttribute('data-theme','dark');</script>
 <script src="/js/core/icons.js" defer></script>
+<script src="/js/core/csrf.js" defer></script>
 <link rel="stylesheet" href="/css/main.css">
 <link rel="stylesheet" href="/css/views/admin.css">
 </head>