fix(app): recover from unavailable project folders

[OpeOginni]

Issue for this PR

Closes #50951

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

• A saved project folder can disappear on Windows or become inaccessible on macOS. Resolving either folder used to fail during Location startup and leave Desktop showing an HTTP 500 error.
• Check the directory before starting the Location. Return a typed error with the exact path: 400 for a missing folder and 403 for denied access (including macOS Unknown/EPERM). Workerd skips the check because it has no local project filesystem.
• In Desktop, check local projects before opening them. Show the path and recovery instructions; if the error page still appears, Go to projects clears the saved route before reloading so it does not reopen the same project.
• The server check prevents the startup failure; the app and Desktop changes give the user a way out. Both cases share that path in this PR.

How did you verify your code works?

• bun run check passed from the repository root.
• Server tests cover missing folders, denied access, restoring access, and workerd. Focused app error/bootstrap and Desktop route-storage tests passed.
• Regenerated the client after changing the Protocol and HttpApi contract.

Screenshots / recordings

• Not captured; these states require an unavailable local project folder.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[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.

[github-actions]

The following comment was made by an LLM, it may be inaccurate: