feat(admin): add box preview and password bypass for administrators

Introduce an `AdminViewBox` handler and route that allows administrators
to view any box directly. If the box is password-protected, the handler
bypasses the protection by setting an unlock cookie with an unlock token
and logs the bypass event.

Additionally, add CSS and JS foundations for a file context menu and
preview actions in the file browser UI.

backend/libs/handlers/app.go

@@ -32,6 +32,7 @@ func (a *App) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("POST /admin/logout", a.AdminLogout)
 	mux.HandleFunc("GET /admin", a.AdminDashboard)
 	mux.HandleFunc("GET /admin/files", a.AdminFiles)
+	mux.HandleFunc("GET /admin/boxes/{boxID}/view", a.AdminViewBox)
 	mux.HandleFunc("POST /admin/boxes/{boxID}/delete", a.AdminDeleteBox)
 	mux.HandleFunc("GET /d/{boxID}", a.DownloadPage)
 	mux.HandleFunc("POST /d/{boxID}/unlock", a.UnlockBox)