chore (frontend): return 404 instead of index.html on requests for non existing assets

[elfedy]

Current index.html references an outdated asset file. On the browser, the error returned was:

 index-Dih_DGnJ.js:1 Failed to load module script: Expected a JavaScript-or-Wasm module script but the server responded with a MIME type of "text/html". Strict MIME type checking is enforced for module scripts per HTML spec.` 

The error makes debugging confusing as the real reason for the failure is that the asset file does not exist. Returning a resource that is not the one requested is also non REST-like behavior and should be avoided when possible.

This PR updates the outdated reference and adds:

• Returning a 404 error when looking for a file in the assets directory.
• A TestFrontend_DistIndexReferencesExistingAssets test to make sure the index does not reference non existing assets

[srikanth-bitdynamics]

Thanks for chasing this down — the MIME type "text/html" diagnosis is spot on, and the /assets/* → 404 change plus the tests are good.

One blocker: the hash bump doesn't fix the root cause, and it's why CI is red.

dist/assets/ is gitignored — only the placeholder index.html is tracked (.gitignore + the assets.go doc comment). So a committed real index.html always references assets that aren't in git; swapping one hash for another just moves the dangling reference. CI never runs npm run build (ci.yml), so TestFrontend_DistIndexReferencesExistingAssets runs against a dist/ with no assets/ and fails on all three refs.

[zheli]

@elfedy Thanks for your contribution.

We have fixed the regression in an earlier PR and you can find it here:
https://github.com/bitdynamics-ab/canton-devkit/pull/241/changes#diff-a3034210ddc9d7e50790c5d3007372044fde9dd7bc0a54bacf1bb3b2845aa89c

I also added a nicer instruction for testing or running UI in dev environment: #255. Plesae check if that helps! I will be happy to approve this PR once the conflicts are fixed.

[elfedy]

@srikanth-bitdynamics @zheli thanks for the comments, makes sense. Removed the regression fix attempt and kept the 404 behavior when serving assets. Will also rename the PR to better reflect the change.

[zheli]

Merged. Thank you for your contribution @elfedy !