Exclude media-free subtrees from the destination picker too

The previous leaf-item fix only caught a single stray file with no
subfolders - it missed a course whose bundled extras (downloaded
source code, a Python virtualenv/dependency dump, project assets)
have plenty of subfolders but no video/audio anywhere in them, which
still left the course itself, and every one of those subfolders,
selectable as destinations. Add a subtree-wide media check (bounded
depth) so a folder with subfolders but no media anywhere beneath it is
excluded the same way, while a folder that still has a real course's
media somewhere inside it - or is simply empty, e.g. a freshly-created
category - stays pickable.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

offlineu_core.py

@@ -1286,6 +1286,44 @@ def _is_unrecognized_leaf_item(directory: Path) -> bool:
     return any(e.is_file() and not e.name.startswith('.') for e in entries)
 
 
+def _subtree_has_any_media(directory: Path, max_depth: int = 8) -> bool:
+    """Whether `directory` or anything under it (bounded depth, so a huge
+    non-course tree can't make a scan slow) contains at least one
+    recognized video/audio file anywhere. False for a folder that's really
+    a course's bundled extras - downloaded source code, a Python
+    virtualenv/dependency dump, a project's asset tree, etc. - which can
+    have plenty of subfolders (so _is_unrecognized_leaf_item doesn't catch
+    it) but no lecture content anywhere in them."""
+    try:
+        entries = list(directory.iterdir())
+    except (PermissionError, OSError):
+        return False
+    for e in entries:
+        if e.is_file() and not e.name.startswith('.') and e.suffix.lower() in VIDEO_EXTENSIONS | AUDIO_EXTENSIONS:
+            return True
+    if max_depth <= 0:
+        return False
+    for e in entries:
+        if e.is_dir() and not e.name.startswith('.') and _subtree_has_any_media(e, max_depth - 1):
+            return True
+    return False
+
+
+def _is_media_free_subtree(directory: Path) -> bool:
+    """A folder that has subfolders (so it isn't just an intentionally
+    empty, freshly-created category) but no video/audio anywhere beneath
+    it - excluded from the destination picker for the same reason as
+    _is_unrecognized_leaf_item, just for a deeper non-course tree instead
+    of a single stray file."""
+    try:
+        has_subdir = any(e.is_dir() and not e.name.startswith('.') for e in directory.iterdir())
+    except (PermissionError, OSError):
+        return False
+    if not has_subdir:
+        return False
+    return not _subtree_has_any_media(directory)
+
+
 def _build_category_index() -> List[Dict[str, Any]]:
     """
     Every category/subcategory folder currently in the library (e.g. IT,
@@ -1305,13 +1343,14 @@ def _build_category_index() -> List[Dict[str, Any]]:
         categories' course titles) can't outweigh a specific match
         elsewhere just by sharing more of it.
     Stops recursing into a folder once it reads as a course itself
-    (_looks_like_course, the same rule the Library browser uses) or as an
+    (_looks_like_course, the same rule the Library browser uses), an
     unrecognized leaf item (_is_unrecognized_leaf_item - an ebook/
-    audiobook/misc file that isn't video or audio, so _looks_like_course
-    doesn't catch it either), so individual courses and standalone files
-    never show up as if they were categories to file things under. The
-    Unsorted folder itself is excluded - it's the source, never a valid
-    destination.
+    audiobook/misc file that isn't video or audio), or a subtree with no
+    video/audio anywhere in it at all (_is_media_free_subtree - a course's
+    bundled source code/project files, a Python virtualenv, etc.), so
+    individual courses and their non-lecture contents never show up as if
+    they were categories to file things under. The Unsorted folder itself
+    is excluded - it's the source, never a valid destination.
     """
     library_root = Path(get_library_root())
     try:
@@ -1334,7 +1373,7 @@ def _build_category_index() -> List[Dict[str, Any]]:
                     continue
             except OSError:
                 pass
-            if _looks_like_course(entry) or _is_unrecognized_leaf_item(entry):
+            if _looks_like_course(entry) or _is_unrecognized_leaf_item(entry) or _is_media_free_subtree(entry):
                 continue
             new_parts = path_parts + [entry.name]
             path_tokens: Set[str] = set()