fix: live preview rebuilds no longer crash the server

When cleaning the previous build, we deleted the entire directory, which
caused problems. Now, we only clean the output dir's children, but leave
the dir itself intact.

src/zona/builder.py

@@ -145,7 +145,13 @@ class ZonaBuilder:
             and self.layout.output.is_dir()
         ):
             logger.debug("Removing stale output...")
-            shutil.rmtree(self.layout.output)
+            # only remove output dir's children
+            # to avoid breaking live preview
+            for child in self.layout.output.iterdir():
+                if child.is_file() or child.is_symlink():
+                    child.unlink()
+                elif child.is_dir():
+                    shutil.rmtree(child)
         logger.debug("Discovering...")
         self._discover()
         logger.debug("Building...")