[explorer/frontend] fix: use runtime configuration on error pages

[cryptoBeliever]

Fixes #2469

Problem

Using the header search bar on a 404 page sends requests to a malformed URL:

❌ https://nem.fyi/undefined/block/B411ECEB...
✅ https://nem.fyi/api/nem/block/B411ECEB...

The requests fail and the search bar silently returns no results.

Root cause

Next.js requires pages/404 and pages/500 to be statically prerendered - it explicitly rejects getServerSideProps and getInitialProps on them. Their HTML, including the window.appConfig script that _app inlines, is therefore generated once, during next build.

PUBLIC_* variables are runtime configuration: the Dockerfile only passes NEXT_PUBLIC_EXPLORER_VARIANT as a build arg. At build time process.env.PUBLIC_API_BASE_URL is undefined, and JSON.stringify drops keys with undefined values entirely - so the key is simply missing from window.appConfig on those pages.

createApiUrl then interpolates it into a template literal:

  `${undefined}/block/HASH`  // → "undefined/block/HASH"

Every other page is server-rendered per request, which is why they are unaffected.

Solution

Serve the configuration from a route that is never statically optimized, and load it from a script tag that runs before the application does.

The inline script is deliberately retained: if /runtime-config.js ever fails to load, server-rendered pages keep working with their freshly inlined values instead of losing configuration entirely.

Because the fix works below the level of any individual request, it covers the search bar, the backend health check, and any request added in the future - no per-caller handling required.

[cryptoBeliever]

@OlegMakarenko, could you please have a look and let me know if this solution makes sense to you or if you'd prefer a different approach?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.87%. Comparing base (03f0aa1) to head (6771ea8).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2483      +/-   ##
==========================================
- Coverage   97.14%   96.87%   -0.27%     
==========================================
  Files         343      320      -23     
  Lines       24477    22212    -2265     
  Branches      221      221              
==========================================
- Hits        23777    21518    -2259     
+ Misses        693      687       -6     
  Partials        7        7              

Flag	Coverage Δ
explorer-frontend	94.59% <ø> (ø)
explorer-puller	?

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