refactor: convert to class-based DI architecture

[djachenko]

Summary

• Setup, Auth, Drive converted to proper classes with dependencies injected via __init__
• New App class as composition root — owns business logic for upload, login, logout
• main.py reduced to thin Typer layer: parses args, delegates to _app = create_app()
• Dependency graph: Config → Setup → Auth → Drive → App
• Ruff upgraded to 0.16, new lint errors fixed, B008 ignore moved to pyproject.toml

Test plan

• test_app.py — new, covers App logic via injected mocks (no patching needed)
• test_main.py — Typer routing only: flags, command dispatch, default command
• test_auth.py, test_drive.py — updated to use injected mocks instead of patch()
• conftest.py — docx_file and drive fixtures; all file creation through create_files
• 47 tests passing, ruff + mypy clean