fix: flatten nested single-child directories after extraction

Archives like Debian_13_VMG.7z contain nested single-child dirs:
  Debian_13_VMG_LinuxVMImages.COM/Debian_13_VMG_LinuxVMImages.COM/

_flatten_nested() collapses these chains until files sit directly
in the parent, eliminating the double-nesting issue.

backend/converter.py

@@ -110,18 +110,38 @@ def extract_if_needed(filename: str) -> Path:
     # If the archive had a single top-level directory matching the stem, use it
     expected_dir = STAGING_IN / stem
     if expected_dir in new_items and expected_dir.is_dir():
-        return expected_dir
+        result = expected_dir
+    else:
+        # Collect scattered files into a subdirectory
+        out_dir = STAGING_IN / stem
+        out_dir.mkdir(parents=True, exist_ok=True)
+        for item in new_items:
+            shutil.move(str(item), str(out_dir / item.name))
+        result = out_dir
 
-    # Otherwise, collect scattered files into a subdirectory
-    out_dir = STAGING_IN / stem
-    out_dir.mkdir(parents=True, exist_ok=True)
-    for item in new_items:
-        dest = out_dir / item.name
-        if item.is_dir():
-            shutil.move(str(item), str(dest))
-        else:
-            shutil.move(str(item), str(dest))
-    return out_dir
+    # Flatten nested single-child directories (archive quirk)
+    result = _flatten_nested(result)
+    return result
+
+
+def _flatten_nested(base: Path) -> Path:
+    """If a directory has exactly one child that is also a directory,
+    move the child's contents up and remove the child. Repeat until stable.
+    Returns the (possibly changed) base path."""
+    changed = True
+    while changed:
+        changed = False
+        if not base.is_dir():
+            break
+        entries = [e for e in base.iterdir() if e.name not in ('.', '..')]
+        if len(entries) == 1 and entries[0].is_dir():
+            inner = entries[0]
+            # Move everything from inner into base
+            for item in list(inner.iterdir()):
+                shutil.move(str(item), str(base / item.name))
+            inner.rmdir()
+            changed = True
+    return base
 
 
 def _detect_archive_ext(name: str) -> Optional[str]: