Plan export start decisions in app core

src/app_core/document_export.h

@@ -26,6 +26,26 @@ struct DocumentExportSuggestedName {
     std::string name;
 };
 
+enum class DocumentExportStartDecision {
+    start_now,
+    show_license_disabled,
+    unavailable_no_canvas,
+};
+
+[[nodiscard]] constexpr DocumentExportStartDecision plan_document_export_start(
+    bool requires_license,
+    bool license_valid,
+    bool has_canvas) noexcept
+{
+    if (requires_license && !license_valid) {
+        return DocumentExportStartDecision::show_license_disabled;
+    }
+
+    return has_canvas
+        ? DocumentExportStartDecision::start_now
+        : DocumentExportStartDecision::unavailable_no_canvas;
+}
+
 [[nodiscard]] inline pp::foundation::Result<DocumentExportFileTarget> make_document_export_file_target(
     std::string_view work_directory,
     std::string_view document_name,