Add NextJS build cache

[cskrov]

The cache spends more time than it saves!

At least for now.

Some numbers from testing

Cache saves 8-10 seconds, depending on the run. Build time typically went from 45 to 36 seconds.
The cache spends around 15-17 seconds (roughly 50-50 restore and save).

[Copilot]

Pull Request Overview

This PR adds steps to cache the NextJS build in the deploy workflow to reduce build time.

• Adds restore and save cache steps for the .next/cache directory.
• Uses file hashes for ts/tsx files to compute the cache key.

Comments suppressed due to low confidence (1)

.github/workflows/deploy-to-dev.yaml:64

• The cache key calculation only includes TypeScript files, which may omit other critical files (e.g., next.config.js, package.json) that influence the NextJS build output. Consider including these files in the hashFiles pattern to ensure the cache is invalidated when relevant changes occur.

key: nextjs-${{ hashFiles('**/*.ts', '**/*.tsx') }}