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/kotlin/de/hoennig/gittally/server/UiController.kt

@@ -87,6 +87,7 @@ class UiController(
                             .jsonName,
                     startedAtIso = build.startedAt.toString(),
                     startedAt = UiFormats.timestamp(build.startedAt),
+                    runningSinceIso = build.runningSince?.toString() ?: "",
                     artifactKey = build.artifactKey,
                     branchUrl = links.branchUrl(build.branch),
                     commitUrl = links.commitUrl(build.commit),