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/seedpass/cli/__init__.py

@@ -28,14 +28,12 @@ no_clipboard_option = typer.Option(
     False,
     "--no-clipboard",
     help="Disable clipboard support and print secrets instead",
-    is_flag=True,
 )
 
 deterministic_totp_option = typer.Option(
     False,
     "--deterministic-totp",
     help="Derive TOTP secrets deterministically",
-    is_flag=True,
 )
 
 # Sub command groups