Separate queue wait from build duration

BuildResult gains runningSince, set when a build leaves the queue; the
recorded duration now measures pure build time from that point, so build
runtimes can be tracked without queue wait. A build cancelled while still
queued records neither. The UI shows the live wait time in italics while
pending and switches to the real build time once the build runs; the Gitea
"after mm:ss" descriptions now also report pure build time.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main/resources/templates/current.html

@@ -10,7 +10,7 @@
             No build is currently running.
         </p>
         <section class="build-card" th:each="build : ${currentBuilds}"
-                 th:attr="data-artifact-key=${build.artifactKey},data-started-at=${build.startedAtIso},data-status=${build.status}">
+                 th:attr="data-artifact-key=${build.artifactKey},data-started-at=${build.startedAtIso},data-running-since=${build.runningSinceIso},data-status=${build.status}">
             <header class="build-card-header">
                 <span th:class="'status status-' + ${build.status}" th:text="${build.status}">running</span>
                 <span class="branch link-tools">