Add billing URL handling for payment required errors

[refoo0]

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[Copilot]

Pull request overview

This PR adds handling for HTTP 402 (Payment Required) responses from the backend API. When a user's organization is inaccessible due to a payment issue, they are now redirected to a configurable billing URL instead of silently redirecting to /.

Changes:

• Detect 402 responses in fetchOrganization and fetchContentTree, throwing a typed HttpError with statusCode: 402
• Add a billingUrl config option backed by BILLING_URL env var (defaults to https://devguard.org)
• Catch the 402 HttpError in the organization layout and redirect to the billing URL

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/data-fetcher/fetchOrganization.ts	Throws HttpError(402) on 402 response; removes stale console.log
src/data-fetcher/fetchContentTree.ts	Adds error checking for non-OK responses including 402 handling
src/config.ts	Adds billingUrl config field with BILLING_URL env var support
src/app/(loading-group)/[organizationSlug]/layout.tsx	Catches 402 HttpError and redirects to billingUrl

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