fix: enable browser executable selection

internal/platform/files/directory_picker.go

@@ -7,3 +7,10 @@ import "context"
 type DirectoryPicker interface {
 	ChooseDirectory(context.Context) (string, error)
 }
+
+// ExecutablePicker opens a platform file chooser for an existing browser
+// executable. The implementation must block only in its own goroutine; UI
+// callers receive the result asynchronously.
+type ExecutablePicker interface {
+	ChooseExecutable(context.Context) (string, error)
+}