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.

USMT-Worker.ps1

@@ -405,6 +405,7 @@ while ($true) {
     Update-WorkerState "Preparing..." 0 0 $globalLastDuration "Calculating..."
 
     $session = Open-ProviderCycle
+    if ($config.IsDemo) { Update-DemoDateTags $dataDir }
 
     $completedList = @()
     if (Test-Path $completedPath) {