Reduce memory usage by ~50 MB for large libraries

FlattenedItem now stores only essential fields (path, name, is_dir, depth)
instead of cloning entire FileTreeNode structures. For 500,000 files, this
reduces memory from ~100 MB to ~50 MB for the flattened view.

- Extract only needed fields in flatten_tree()
- Add find_node_by_path() helper to look up full nodes when needed
- Update all UI and state code to use new structure

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cm-player"
-version = "0.1.23"
+version = "0.1.24"
 edition = "2021"
 
 [dependencies]