fix(model): handle nested directories in model file removal

[erning]

Summary

remove_model_files() used a shallow read_dir that only iterated one directory level and skipped subdirectories. Model manifests can specify nested file paths (e.g. subdir/weights.bin), and the download logic creates those subdirectories via create_dir_all. When a user deleted a model, files inside subdirectories were silently left behind — the UI reported success but disk space was not freed.

• Add remove_dir_all handling for subdirectories encountered during removal
• Add count_files_recursive helper to maintain accurate file count reporting
• Add a test that creates a model directory with nested files and verifies full cleanup

Test plan

• cargo test --workspace passes (including new remove_model_files_handles_subdirectories test)
• cargo clippy --workspace --all-targets clean
• Download a model whose manifest contains nested paths, remove it, verify no residual files

[missuo]

LGTM. Clean fix with a good test-first approach.

• count_files_recursive correctly counts files before remove_dir_all for accurate reporting
• Test covers the exact scenario (nested subdirectories in model manifest)
• One minor thought: count_files_recursive silently returns 0 on read errors — fine for a count helper, just noting it