Modernize JSTL taglib URIs to jakarta.tags.* (post-jakarta migration)

Background

As part of the javax.* → jakarta.* migration (#861), the JSTL dependency moves to JSTL 3.0 (org.glassfish.web:jakarta.servlet.jsp.jstl:3.0.1). JSTL 3.0 introduces new canonical taglib URIs (jakarta.tags.core, jakarta.tags.fmt, …) replacing the legacy http://java.sun.com/jsp/jstl/* URIs.

This is not urgent / not a blocker. Verified from the TLDs inside jakarta.servlet.jsp.jstl-3.0.1.jar: the impl declares both the new jakarta.tags.* URIs and the legacy http://java.sun.com/jsp/jstl/* URIs, so the existing JSPs continue to resolve and render correctly. This is purely modernization / future-proofing.

Scope

The legacy URIs are declared in a single file — src/main/webapp/translation.jsp — which the other JSTL-using JSPs pull in via <%@ include file="translation.jsp" %>. So the change is two lines:

- <%@ taglib prefix="c"   uri="http://java.sun.com/jsp/jstl/core"%>
- <%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt"%>
+ <%@ taglib prefix="c"   uri="jakarta.tags.core"%>
+ <%@ taglib prefix="fmt" uri="jakarta.tags.fmt"%>

Consumers (inherit via static include, no change needed): index.jsp, login.jsp, register.jsp, setup.jsp, getStarted.jsp, readyToPlay.jsp, blockedMessage.jsp.

Acceptance criteria

• translation.jsp uses the jakarta.tags.* taglib URIs.
• Translation rendering (fmt:setLocale / fmt:setBundle / fmt:message) still works — verify a translated page renders in a non-English locale.

Notes

• Do after the jakarta migration (#861) lands, not as part of the build-config PRs.
• Relates to the JSP/JSTL translation render path documented in docs/i18n-findings.md.

Refs #861.

Start with src/main/webapp/translation.jsp, where the c and fmt taglib declarations are defined for the included JSP pages. Replace the two legacy URIs with jakarta.tags.core and jakarta.tags.fmt, then verify that a translated page renders correctly in a non-English locale using the fmt translation path.