feat: project tree delete, close tab, Linux AppImage bundling

- Right-click delete on classes in Project Tree dock
- File > Close (Ctrl+W) to unload active project tab
- File > Open now replaces current project instead of merging
- Linux CI builds AppImage via linuxdeploy + Qt plugin so users
  don't need Qt installed (fixes libQt6Core.so.6 not found)
- Pin ubuntu-22.04 for broader glibc compatibility

src/mainwindow.h

@@ -31,7 +31,7 @@ private slots:
     void openFile();
     void saveFile();
     void saveFileAs();
-
+    void closeFile();
 
     void addNode();
     void removeNode();