fix: share branch session launch state

[matt2e]

Summary

• Move branch pending session launch state into a shared helper used by branch cards and diff views.
• Let diff-viewer note and commit launches create shared pending timeline rows and keep queued or running status synced.
• Carry project and repo context through diff routes for launched sessions.

Testing

• Pre-push hooks passed during git push: crates-fmt, crates-lint, crates-test, staged-ci, differ-ci.