i18n(ja): Update guides/authentication.mdx

[jp-knj]

Description (required)

This PR completes the Japanese translation of the Authentication guide and brings it fully in line with the current English source.(see: https://github.com/withastro/docs/commits/main/src/content/docs/en/guides/authentication.mdx?since=2024-04-05T18:09:21.000Z)

Key updates

• Added missing sections: Better Auth, Clerk, Community Resources, plus the UIFrameworkTabs code example.
• Unified glossary terminology (例: アダプター for “adapter”, インテグレーション for “integration”) across the entire page.
• Re-applied JP style guide: “です・ます調”, removed redundant phrases, and eliminated half-width spaces between Japanese and Latin text.
• Harmonised file names (auth.config.ts vs .mjs) and added an explanatory note where the extension differs from the EN source.
• Localised UI strings inside code blocks (“Login” → “ログイン”, “Logout” → “ログアウト”, etc.) for consistency.

Related issues & labels (optional)

• Closes #
• Suggested label: i18n

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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.