[bps]

Description

Calibre's Preferences → Plugins → Customize entry point calls do_user_config() with only the parent GUI. Unlike the toolbar entry point, it does not pass the running plugin action. This left every column dropdown at “(Not mapped)” and could erase all eight saved mappings when the dialog was accepted.

• Fall back to Calibre's loaded interface action when no action is supplied explicitly.
• Pass the loaded action through the config_widget() hook as well.
• Add regression tests using the real plugin and configuration logic with mocked Calibre/Qt boundaries. Cover column discovery, saved mappings, explicit-action precedence, and cancellation.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

Verification

• Before the fix: three regression cases failed (Preferences column discovery, config_widget() column discovery, and Preferences mapping preservation); the toolbar and cancellation cases passed.
• After the fix: all six regression cases pass.
• Full suite: 501 passed, 23 live-API integration tests skipped. No real-library or API writes were performed.
• Ruff lint and formatting checks pass.
• Pyright reports six existing errors in unchanged files; the same errors were verified against unmodified HEAD before this commit.
• No interactive Calibre UI reproduction was performed.

Checklist

• Tests pass (full local suite; live-API integration tests skipped)
• Code is formatted
• Documentation reviewed; no update needed

Related issues

Fixes #34

Reporter workaround: #1 (comment)