Improved docx and odt import (#890)

Co-authored-by: Thorsten Sommer <SommerEngineering@users.noreply.github.com>

app/MindWork AI Studio/Tools/SlideTextContent.cs

@@ -4,5 +4,10 @@ namespace AIStudio.Tools;
 
 public sealed class SlideTextContent(string textContent) : ISlideContent
 {
-    public StringBuilder Text => new(textContent);
+    //
+    // One builder per slide, created once: an expression-bodied property would hand out a fresh
+    // builder on every access, so appending further text to a slide would write into a throwaway
+    // object and the text would never reach the slide.
+    //
+    public StringBuilder Text { get; } = new(textContent);
 }