Bump Next.js to version 15.4.0

[conico974]

Update Next.js dependency to version 15.4.0 and add a local GitBook URL for development.

We can't go above this version for now.
15.4.1+ will cause all suspense boundary to trigger even on ISR page.
15.5+ cause every ISR route to become PPR, with every suspense boundary triggering everytime as well.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 46d14b8

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

Summary of the deployments:

Version	URL	Status
Vercel	https://gitbook-v2-3bpiqyk05-gitbook.vercel.app	✅
Cloudflare	https://130d899d-gitbook-open-v2-preview.gitbook.workers.dev	✅

Test content

Site	2v	2c
GitBook	https://gitbook-v2-3bpiqyk05-gitbook.vercel.app/url/gitbook.com/docs	https://130d899d-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.com/docs
E2E	https://gitbook-v2-3bpiqyk05-gitbook.vercel.app/url/gitbook.gitbook.io/test-gitbook-open	https://130d899d-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.gitbook.io/test-gitbook-open

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Build	Status	Details	Updated (UTC)
customers-v2 (Inspect)	👍 Changes approved	3 changed	Sep 12, 2025, 9:11 AM
v2-cloudflare (Inspect)	👍 Changes approved	1 changed	Sep 12, 2025, 9:15 AM
v2-vercel (Inspect)	👍 Changes approved	1 changed	Sep 12, 2025, 9:15 AM