On Windows, if the user clicks the Tailscale icon in the system tray,
it opens a login URL in the browser.
When the login URL is opened, `state/nonce` cookies are set for that particular URL.
If the user clicks the icon again, a new login URL is opened in the browser,
and new cookies are set.
If the user proceeds with auth in the first tab,
the redirect results in a "state did not match" error.
This patch ensures that each opened login URL sets an individual cookie
that remains valid on the `/oidc/callback` page.
`TestOIDCMultipleOpenedLoginUrls` illustrates and tests this behavior.
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