Guard unavailable create PR actions

[Emanuele-web04]

Summary

• route direct create-pr attempts through the same availability check as the header action
• prevent stale post-push toasts from offering Create PR when there is no branch diff
• cover default-upstream and published feature-branch cases in git action logic tests

Verification

• bun run vitest run src/components/GitActionsControl.logic.test.ts
• bun run build
• git diff --check