Upgrade mithril explorer to Next 15

[Alenar]

Content

This PR upgrade mithril explorer main dependencies, NextJs, to its latest major version.

There's some caveat:

• Vercel, the NextJs editor, recommend to use React 19 that's still a release candidate, but it's not compatible with @reduxjs/toolkit so React 18.3 is kept.
• Version 15 of eslint-config-next is compatible with eslint 9 but still ask for version 8.57.1 in its package json, leading to warnings when using npm install. We should keep using eslint 8.57 until this is solved.
• Vercel say that Turbopack, its webpack replacement made with rust, is ready for use in dev, but when we use it the app crash because it can't import properly the wasm library. We should keep using webpack until this is solved. Note: To test this tool, add --turbopack to the dev scripts in package.json.

Pre-submit checklist

• Branch
  • [x] Crates versions are updated (if relevant)
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
• PR
  • [x] No clippy warnings in the CI
  • [x] Self-reviewed the diff
  • [x] Useful pull request description
  • [x] Reviewer requested

[github-actions[bot]]

Test Results

    4 files  ±0     51 suites  ±0   10m 38s ⏱️ +18s 1 418 tests ±0  1 418 ✅ ±0  0 💤 ±0  0 ❌ ±0  1 629 runs  ±0  1 629 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit ceec2e7a. ± Comparison against base commit dad3dbc0.