feat: Move integrations to separate page, add testing email API

[aXenDeveloper]

Improving Documentation

• Run pnpm lint:fix to fix formatting issues before opening the PR.
• Read the Docs Contribution Guide: https://vitnode.com/docs/dev/contribution

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Project	Deployment	Actions	Updated (UTC)
vitnode-docs	Ready	Preview, Comment	Jul 4, 2026 5:27pm

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[aXenDeveloper]

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Code Review

This pull request introduces a new "System" section in the admin panel, moving the "Integrations" view there from the "Debug" panel and adding a new feature to send test emails to verify email configurations. It also updates cron parser imports to use CronExpressionParser. Feedback focuses on improving error handling for the new test email feature, specifically recommending that the frontend form submission handle network/runtime errors and display specific API errors, and that the backend API route verify the email service's initialization and catch SMTP/sending errors gracefully.

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