Fixes #3139 

## Summary
- Fixes the bug where clicking **Sync to GitHub** on the Publish tab and
navigating away before the push finishes dropped the in-flight state —
the push actually continued in the main process, but on return the UI
showed neither success nor failure.
- Moves sync state (`isSyncing`, `syncError`, `syncSuccess`,
`conflicts`, rebase status, etc.) out of local `useState` in
`ConnectedGitHubConnector` and into a Jotai atom keyed by `appId`
(`src/atoms/githubSyncAtoms.ts`), so it survives unmount/remount.
- Adds success/error toasts on completion so users get feedback even
when they're viewing a different preview tab.

## Test plan
- [x] New unit tests in `src/atoms/githubSyncAtoms.test.tsx` verify
state is preserved across unmount/remount, that a "completion" happening
while unmounted is visible on remount, that state is isolated per
`appId`, and that `null` appId is a no-op.
- [x] `npm run ts` — clean
- [x] `npm run fmt` / `npm run lint:fix` — clean (only the pre-existing
unrelated warning in `local_agent_handler.test.ts`)
- [x] `npm test` — 1003/1003 passing
- [ ] Manual: click Sync to GitHub, immediately switch to Code tab,
switch back — spinner should still show (if still running) or the
success message should be visible (if finished); a toast should appear
on completion regardless of tab.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
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Co-authored-by: Will Chen <7344640+wwwillchen@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>