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/docs.js

@@ -295,6 +295,7 @@
                 jsPDF: { unit: 'mm', format: 'a4', orientation: 'portrait' },
                 pagebreak: { mode: ['css', 'legacy'], avoid: ['pre', 'table', 'img', 'blockquote', 'h1', 'h2', 'h3', 'h4', 'li', 'tr'] }
             };
+            await loadHtml2Pdf();
             await html2pdf().set(opt).from(page).toPdf().get('pdf').then(pdf => {
                 const n = pdf.internal.getNumberOfPages();
                 const w = pdf.internal.pageSize.getWidth(), h = pdf.internal.pageSize.getHeight();
@@ -382,8 +383,16 @@
         };
         addToolbarButtons();
 
+        // The search index fetches every article. On a first visit to any other page it waits
+        // until the device data has drawn, so those requests don't hold the dashboard up.
+        let indexReady = location.pathname.startsWith('/docs');
+        const indexNow = () => { if (indexReady) return; indexReady = true; reindex(); };
+        document.addEventListener('usmt:data', () => setTimeout(indexNow, 300), { once: true });
+        setTimeout(indexNow, 4000);
+        wrap('switchPage', id => { if (id === 'docs') indexNow(); });
+
         wrap('loadDocsList', () => {
-            reindex();
+            if (indexReady) reindex();
             if (firstLoad) {
                 firstLoad = false;
                 const t = titleFromPath();