docs(firebase): set maxAge on session cookie in signin example

[sharadvc]

Summary

The Firebase auth signin example created a 5-day session cookie via expiresIn but did not set maxAge on Astro's cookies.set(), so browsers treated __session as a session cookie and cleared it on tab close.

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

Test plan

• Diff is English guide only (src/content/docs/en/guides/backend/firebase.mdx)
• Confirm snippet renders on the Firebase backend guide preview

Closes #14526

[astrobot-houston]

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Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[ArmandPhilippot]

See #14541 (comment)