i18n(ja): update guides/backend/firebase.mdx

[yamotech]

Description (required)

Apply changes from #14534 to Japanese docs.

References

• Related to docs: set Firebase session cookie maxAge to match expiresIn #14534

[astrobot-houston]

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Tracked Files

File	Note
ja/guides/backend/firebase.mdx	Localization changed, will be marked as complete.

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.

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[yanthomasdev]

LGTM! Thanks!