fix(desktop): narrow existing state detection (#51323)

[kvnloo]

Summary

Fork tip leaf for origin anomalyco#51323.

hasExistingAppState only treats legacy default.dat and opencode.*.dat store files as prior install state. Unrelated .dat files/directories no longer mark a fresh desktop install as existing.

Proof

cd packages/desktop && bun test src/main/install-state.test.ts
# GREEN: 2 pass / 13 asserts
# negative (legacy endsWith(".dat")): 1 fail / 1 pass
# restore: 2 pass

github_writes_origin: 0. Codex promote parked.

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.