Refactor test fixtures and fix linting issues

- Moved `client` and `dummy_nostr_client` fixtures to `src/tests/conftest.py` to fix F811 redefinition errors and improve test organization.
- Introduced `make_dummy_nostr_client` factory fixture to support tests requiring multiple client instances.
- Updated all affected tests to use the new fixture pattern.
- Removed deprecated `is_flag=True` from Typer boolean options in CLI modules.
- Fixed F541 (f-string without placeholders) in `entry_management.py`.
- Fixed F841 (unused variable) in `manager.py`.

Co-authored-by: PR0M3TH3AN <[email protected]>

src/tests/test_api_profile_stats.py

@@ -1,4 +1,3 @@
-from test_api import client
 import pytest