Show the queued show and its start time in the index.php sidebar

- Shows get a description and an icon upload; broadcasts get a scheduled_at
  start time. schema.sql adds the columns to existing databases and seeds
  the descriptions from the old hand-written sidebar.
- The dashboard and cli/set_show.php take an optional date and Eastern start
  time when queuing; index.php shows it with the UTC conversion.
- liveBroadcast() and currentShow() ignore broadcasts unseen for longer than
  the grace period, so a stopped poller can't leave a show stuck as live.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

includes/admin_layout.php

@@ -38,6 +38,17 @@ function adminHeader(string $title, ?array $admin = null): void {
 		        #header ul.admin-nav a {
 		            color: #fff;
 		        }
+		        /* Strata doesn't style these input types */
+		        input[type="date"],
+		        input[type="time"] {
+		            background: #f7f7f7;
+		            border-radius: 0.35em;
+		            border: solid 2px transparent;
+		            color: inherit;
+		            height: 2.75em;
+		            padding: 0 0.75em;
+		            width: 100%;
+		        }
 		        .admin-message {
 		            padding: 0.75em 1em;
 		            border-radius: 4px;