fix: split pending kanban idle sessions

[laxmanclo]

Summary

• rename the first Kanban column to Pending
• split pending sessions into Working and Idle sections
• show Idle as its own card badge and cover the behavior in the PR hydration integration test

What this fixes

Idle sessions were mixed into the active working list, making the Pending column look noisier and hiding which sessions were actually waiting.

Verification

• npm test -- src/renderer/tests/integration/pr-hydration.test.tsx
• go test ./...