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/entry.py

@@ -123,7 +123,7 @@ def entry_add(
     username: Optional[str] = typer.Option(None, "--username"),
     url: Optional[str] = typer.Option(None, "--url"),
     no_special: bool = typer.Option(
-        False, "--no-special", help="Exclude special characters", is_flag=True
+        False, "--no-special", help="Exclude special characters"
     ),
     allowed_special_chars: Optional[str] = typer.Option(
         None, "--allowed-special-chars", help="Explicit set of special characters"
@@ -137,7 +137,6 @@ def entry_add(
         False,
         "--exclude-ambiguous",
         help="Exclude ambiguous characters",
-        is_flag=True,
     ),
     min_uppercase: Optional[int] = typer.Option(None, "--min-uppercase"),
     min_lowercase: Optional[int] = typer.Option(None, "--min-lowercase"),