Add Swagger UI for API Challenges

[eviltester]

Summary

• Adds Swagger UI pages backed by generated OpenAPI JSON for API Challenges and the prefixed practice modes.
• Makes Swagger UI use the current request host first so local docs call local APIs by default.
• Forces the embedded Swagger UI into the API Challenges light theme.
• Fixes Javalin static asset serving so CSS, JS, favicon, and image assets are not swallowed by generic fallback routes.

Validation

• mvn -pl challenger -am -Dtest=UiPagesAreReachableTest -DfailIfNoTests=false test
• mvn -pl thingifier -DskipTests checkstyle:check@project-fqn-check
• mvn spotless:check
• mvn -pl challenger -am -DskipTests package

Closes #61

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve the externally visible scheme for OpenAPI servers

When the application is deployed behind an HTTPS-terminating reverse proxy, request.scheme() is the backend connection scheme (normally http), while the browser is on https. This places http://<public-host> first in the generated servers list, so Swagger UI's Try it out requests become mixed-content requests and are blocked by browsers. Derive the public origin from a trusted forwarded-proto configuration/header or retain the configured HTTPS server instead.

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