fix read-only viewer

src/ttfrog/themes/default/static/froghat.js

@@ -126,9 +126,13 @@ class Froghat {
     }
 
     getMarkdown() {
+        if (!this.cachedMarkdown) {
+            this.cachedMarkdown = this.element.textContent;
+        }
         return this.cachedMarkdown;
     }
 
+
     view() {
         /*
          * Convert the wiki read-only mode and display the current HTML.