fix(app): fade completed library activity

[darwin67]

Summary

• fade the library activity track and bar to transparent over roughly 300 ms after work completes
• restore full visibility whenever loading, discovery, import, filtering, or debounced search activity starts
• keep repeated search input from resetting progress while preserving stale-debounce protection
• add regressions for fade duration, activity interruption, search debounce visibility, and activity entry points

Verification

• cargo test -p shosai-app (304 tests)
• cargo clippy -p shosai-app --all-targets -- -D warnings
• cargo fmt --all -- --check