Fix Vorlagen editor: drop Summernote, use code editor for all templates (STI-82)

Summernote WYSIWYG was mangling Django template syntax ({{ }}, {% %})
on save, causing content to revert to corrupted state. Switched all
template types to the plain code editor textarea which preserves
content exactly as-is.

Also removed jQuery/Summernote JS dependencies from the editor page,
and fixed getEditorContent reference in preview code.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/stiftung/views/vorlagen.py

@@ -57,10 +57,9 @@ def vorlage_editor(request, pk):
     html_json = json.dumps(vorlage.html_inhalt)
     html_json = html_json.replace("<", "\\u003c").replace(">", "\\u003e")
 
-    # Serienbrief templates are full HTML documents with Django template tags
-    # ({% for %}, {% if %}) — Summernote WYSIWYG mangles these.
-    # Use a plain code editor textarea instead.
-    use_code_editor = vorlage.kategorie == "serienbrief"
+    # All templates contain Django template tags ({{ }}, {% %}) that
+    # Summernote WYSIWYG mangles on save. Use plain code editor for all.
+    use_code_editor = True
 
     return render(request, "stiftung/vorlage_editor.html", {
         "vorlage": vorlage,