Update dependencies and Next.js

[RaananW]

Summary

• refresh compatible direct and transitive dependencies
• update the Next.js toolchain to 16.3.1, the newest release allowed by the repository's seven-day release-age policy
• add the generated Next.js TypeScript environment reference
• override deepmerge-ts to its secure v8 release
• update the npm lockfile

Validation

• npm test -- --reporter=dot
• npm audit --audit-level=high
• npm run build

[bjsplat]

Documentation snapshot uploaded.

Snapshot: https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/DOCS/refs/pull/1605/merge
Path: DOCS/refs/pull/1605/merge
Build: https://dev.azure.com/babylonjs/ContinousIntegration/_build/results?buildId=57483

[bjsplat]

Documentation snapshot uploaded.

Snapshot: https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/DOCS/refs/pull/1605/merge
Path: DOCS/refs/pull/1605/merge
Build: https://dev.azure.com/babylonjs/ContinousIntegration/_build/results?buildId=58067