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/lib/dialogs.ts

@@ -0,0 +1,42 @@
+import { Dialogs } from "@wailsio/runtime";
+
+import { isWindows } from "@/lib/platform";
+
+// Derived from the runtime rather than deep-imported: the package's exports map
+// only exposes the types barrel, not "@wailsio/runtime/types/dialogs".
+type MessageDialogOptions = Parameters<typeof Dialogs.Error>[0];
+
+// On Windows a native MessageBox attached to a parent window disables that
+// parent (WS_DISABLED) for the lifetime of the dialog and re-enables it on
+// dismissal. When the parent is the main window — whose WindowClosing hook
+// hides instead of closes (main.go) — the enable/hide sequence can race and
+// leave the window unable to process its close (X) button afterwards: the user
+// reports the main window can no longer be closed once an error dialog (e.g. a
+// rejected login) has been shown. Detaching the dialog gives the MessageBox a
+// NULL owner, so no window is ever disabled and the X keeps working.
+//
+// macOS keeps the attached (sheet-style) presentation — the bug is Windows-only
+// and detaching there loses the sheet animation — so we only force Detached on
+// Windows and leave any caller-supplied value untouched elsewhere.
+function withDetached(options: MessageDialogOptions): MessageDialogOptions {
+    if (options.Detached !== undefined || !isWindows()) {
+        return options;
+    }
+    return { ...options, Detached: true };
+}
+
+export function errorDialog(options: MessageDialogOptions): Promise<string> {
+    return Dialogs.Error(withDetached(options));
+}
+
+export function warningDialog(options: MessageDialogOptions): Promise<string> {
+    return Dialogs.Warning(withDetached(options));
+}
+
+export function infoDialog(options: MessageDialogOptions): Promise<string> {
+    return Dialogs.Info(withDetached(options));
+}
+
+export function questionDialog(options: MessageDialogOptions): Promise<string> {
+    return Dialogs.Question(withDetached(options));
+}