perf: batch preset skill toggles

[vipinchan]

Summary

• Add a batched preset membership toggle command so multi-select enable/disable uses one IPC call.
• Apply all membership changes in a single database transaction.
• Rewrite metadata once and refresh the tray once per batch instead of once per skill.
• Keep behavior membership-only, matching the existing single-skill add/remove commands.

Why

The current multi-select preset action loops over selected skills and performs N sequential IPC calls. This change reduces that to one backend call and one metadata write while preserving semantics.

This addresses the batch operation portion of #295.

Test plan

• npm run build
• cargo test --manifest-path src-tauri/Cargo.toml
• Manually enable and disable multiple selected skills in a preset and verify counts and membership refresh correctly.

Context

Split from #289 per maintainer feedback. The sync-time same-name dedup changes are intentionally not included.