Improve flashgrep lifecycle and workspace activation

[wgqqqqq]

Summary

• switch BitFun flashgrep integration to a managed stdio backend and clean up the flashgrep module layout
• make workspace activation stop blocking on flashgrep and other warmup work during fast workspace switching
• make app close stop waiting on flashgrep shutdown, and update index status wording so tracked changes are not treated as stale by default

Testing

• cargo fmt --all
• cargo check -p bitfun-desktop
• cargo test -p bitfun-desktop
• cargo check --workspace