[fix](trx-frontend-http): improve scheduler entry controls

Add previous/next scheduler entry controls for overlapping\ntime-span slots and fix interleave timing calculations so\nthe active slot and countdown align with the overlap window.\n\nVerification: cargo test -p trx-frontend-http scheduler\nVerification: node --check assets/web/plugins/scheduler.js\n\nCo-authored-by: OpenAI Codex <codex@openai.com>

Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/style.css

@@ -396,6 +396,14 @@ input.status-input, select.status-input { width: 100%; padding: 0.45rem 0.5rem;
   border-color: var(--accent-yellow);
   color: var(--accent-yellow);
 }
+.scheduler-step-controls {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.35rem;
+}
+.scheduler-step-controls button {
+  min-width: 7.6rem;
+}
 .scheduler-release-status {
   color: var(--text-muted);
   font-size: 0.78rem;