docs: set Firebase session cookie maxAge to match expiresIn

[mrchatam]

Description (required)

In the English Firebase auth sign-in example, createSessionCookie() is given a 5-day expiresIn, but cookies.set() only sets path. Without maxAge/expires, the browser treats __session as a session cookie and drops it when the tab closes.

Adds maxAge: fiveDays / 1000 so the browser cookie lifetime matches the Firebase token. English only.

References

• Closes Firebase auth example - Cookie expiration clarification #14526

[astrobot-houston]

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[ArmandPhilippot]

Your previous PR was a typo fix not affecting other locales, so I have added [i18nIgnore]. Here, we want all locales to update the code snippet, so I removed [i18nIgnore] from the title.

But, thanks, LGTM!