feat!: upgrade to next 16

[MikaelSiidorow]

Description

Upgrade Next.js from 15.5.9 to 16.1.6 and related dependencies.

Changes

• Next.js 15.5.9 → 16.1.6 (Turbopack now default bundler)
• React 19.2.3 → 19.2.4
• PayloadCMS ^3.55.1 → ^3.72.0 (required for Next.js 16 compatibility)
• middleware.ts → proxy.ts — Next.js 16 renames middleware to proxy to clarify its network boundary role
• Removed --turbopack flags from scripts (Turbopack is default in v16)
• Removed eslint config from next.config.mjs (no longer supported in v16)
• Enabled Turbopack filesystem caching for dev (turbopackFileSystemCacheForDev)
• Guard generateImageMetadata against undefined params during build (Next.js 16 probes route structure with undefined params in the "collecting page data" phase)
• Restored catalog: references for react/react-dom/@types that the codemod pinned to exact versions

Peer dependency warnings (non-blocking)

• react-moment, react-live-clock, @tietokilta/ilmomasiina-client declare react@^18 peer but work with React 19

Before submitting the PR, please make sure you do the following

• If your PR is related to a previously discussed issue, please link to it here.
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Make sure the commit history is linear, up-to-date with main branch and does not contain any erroneous changes

Formatting and linting

• Format code with pnpm format and lint the project with pnpm lint