fix: prevent double-nested directory on archive extraction

Archives like 64bit.7z that already contain a 64bit/ top-level dir
were extracted into STAGING_IN/64bit/, creating 64bit/64bit/ nesting.

Now extract into STAGING_IN root, then:
- If the archive produced a single dir matching the stem, use it
- Otherwise, collect scattered files into a stem-named subdirectory

Removes the need for nested path: 64bit/Debian.vmdk instead of
64bit/64bit/Debian.vmdk

backend/converter.py

@@ -75,13 +75,34 @@ def extract_if_needed(filename: str) -> Path:
         if stem.endswith(e):
             stem = stem[:-len(e)]
             break
-    out_dir = STAGING_IN / stem
-    out_dir.mkdir(parents=True, exist_ok=True)
 
-    result = subprocess.run(cmd, cwd=str(out_dir), capture_output=True, text=True, timeout=600)
+    # Snapshot STAGING_IN before extraction
+    before = set(STAGING_IN.iterdir()) if STAGING_IN.exists() else set()
+
+    # Extract into STAGING_IN root (not a subdir) to avoid double nesting
+    # when the archive already has a top-level directory
+    result = subprocess.run(cmd, cwd=str(STAGING_IN), capture_output=True, text=True, timeout=600)
     if result.returncode != 0:
         raise RuntimeError(f"{tool} failed: {result.stderr.strip()[:500]}")
 
+    # Determine what was created
+    after = set(STAGING_IN.iterdir())
+    new_items = after - before
+
+    # 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
+
+    # 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