Faster first load; demo date tags stay current

First load:
- Draw the Overview as soon as the data arrives instead of on its next
  2-second timer tick (about 2.5 s to 0.6 s on a typical connection).
- Start on DOMContentLoaded instead of window.onload.
- Load the ~900 KB PDF library only when someone exports a PDF.
- Fetch the startup files side by side, and build the docs search index
  after the dashboard has drawn (unless the visit starts on /docs).
- Show a loading placeholder on the Overview until the data arrives.

Demo: the date-batch tags sit on fixed working-day offsets from today.
The seed records them in Demo-Dates.json and the worker relabels them when
the day changes, so a long-running container never looks stale.

web/overview.css

@@ -164,3 +164,13 @@ table.ov-heat td { padding: 0; text-align: center; border: 0; background: none;
 
 /* Undocumented apps list: room goes to the device names */
 #ov-table td.ov-src-list { white-space: normal; overflow-wrap: anywhere; }
+
+/* Loading placeholder (first fraction of a second, before the data arrives) */
+.ov-skel { min-height: 150px; margin-bottom: 18px;
+    background: linear-gradient(90deg, var(--bg-card) 25%, rgba(127,127,127,0.10) 50%, var(--bg-card) 75%);
+    background-size: 200% 100%; animation: ov-skel 1.2s linear infinite; }
+.ov-skel-hero { min-height: 120px; }
+.ov-skel-row { display: grid; grid-template-columns: 1fr 1fr; gap: 18px; }
+@media (max-width: 700px) { .ov-skel-row { grid-template-columns: 1fr; } }
+@keyframes ov-skel { from { background-position: 100% 0; } to { background-position: -100% 0; } }
+@media (prefers-reduced-motion: reduce) { .ov-skel { animation: none; } }