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/js/features/sharing/fileSharing.js

@@ -5,11 +5,9 @@
  */
 
 const fileSharing = {
-    /** Auth header helper */
+    /** Auth header helper — tokens are in HttpOnly cookies now */
     _headers(json = true) {
-        const h = {};
-        const token = localStorage.getItem('oxicloud_token');
-        if (token) h['Authorization'] = `Bearer ${token}`;
+        const h = { ...getCsrfHeaders() };
         if (json) h['Content-Type'] = 'application/json';
         return h;
     },