fix(ui): keep main window closable after a native dialog on Windows (#6319)

A native Windows MessageBox attached to a parent window disables that
window (WS_DISABLED) for its lifetime and re-enables it on dismissal.
When the parent is the main window — whose WindowClosing hook hides
instead of closes — the enable/hide sequence races and leaves the window
unable to process its close (X) button afterwards, so e.g. a rejected
login error dialog left the main window stuck open.

Route all native dialogs through src/lib/dialogs.ts, which forces
Detached: true on Windows (NULL owner, no window ever disabled) and is a
no-op on macOS/Linux (keeps the attached sheet-style presentation).

client/ui/frontend/src/hooks/useManagementUrl.ts

@@ -1,5 +1,5 @@
 import { useEffect, useRef, useState } from "react";
-import { Dialogs } from "@wailsio/runtime";
+import { warningDialog } from "@/lib/dialogs.ts";
 import i18next from "@/lib/i18n";
 import { useSettings } from "@/contexts/SettingsContext.tsx";
 
@@ -70,7 +70,7 @@ export function useManagementUrl() {
             switchConfirmOpenRef.current = true;
             const cancelLabel = i18next.t("common.cancel");
             const confirmLabel = i18next.t("settings.general.management.switchCloudConfirm");
-            void Dialogs.Warning({
+            void warningDialog({
                 Title: i18next.t("settings.general.management.switchCloudTitle"),
                 Message: i18next.t("settings.general.management.switchCloudMessage"),
                 Buttons: [