Handle 404 when importing a valid-looking but nonexistent issue URL

[richardkmichael]

When a user imports a URL like github.com/owner/repo/issues/99999 that returns 404 from the REST API, the popup should handle this gracefully.

Currently, the import validation accepts URLs that match the issue URL pattern, but doesn't verify the issue exists until fetch time. A 404 response likely shows as a generic failed load.

Consider showing a clear message that the issue was not found, distinct from network errors or rate limiting.