Mamba 1.4.5-hotfix

Fixes repeated “Mamba is busy” errors when loading large game libraries.

• Queue cover-art requests before they reach the background worker, with up to four requests at once.
• Share duplicate artwork requests instead of submitting them repeatedly.
• Correct broken separators in Activity text.

Validation: all 188 tests pass, including a 200-cover request regression test. Windows installer and Linux AppImage / Debian packages are built from the same source commit.