fix: accurate rebase status detection and continuous status polling

[AmethystLiang]

Problem

During rebasing, the UI sometimes leaves stale 'Rebasing' badges behind because it relies on checking REBASE_HEAD which is only present during conflicting steps of a rebase, and can sometimes be left behind even after the rebase finishes. Additionally, when the right sidebar is unmounted, worktree git status stops polling, preventing worktree cards from updating their states correctly.

Solution

1. Changed rebase detection to exclusively check for rebase-merge/ or rebase-apply/ directories, which exist for the entire duration of a rebase operation.
2. Moved the useGitStatusPolling() hook from RightSidebar up to App.tsx so it can continue to poll in the background even when the sidebar is closed.