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.js

@@ -30,6 +30,7 @@
 
     let state = { kind: null, arg: null, map: 'left' };
     let lastSig = '';
+    let loaded = false;   // set once the dashboard's first data load has finished
 
     // ---------------------------------------------------------------------------------
     //  Data
@@ -222,9 +223,16 @@
         return c;
     }
 
+    // Shown from the moment the page opens until the first data arrives
+    function renderSkeleton(root) {
+        root.innerHTML = '<div class="ov-head"><div><h1 class="ov-title">Overview</h1><p class="ov-subtitle">Loading the fleet...</p></div></div>'
+            + '<section class="ov-card ov-skel ov-skel-hero"></section><div class="ov-skel-row"><section class="ov-card ov-skel"></section><section class="ov-card ov-skel"></section></div>';
+    }
+
     function render() {
         const root = document.getElementById('overview-root');
         if (!root) return;
+        if (!loaded) { renderSkeleton(root); return; }
         const all = rows();
         hideTip();
 
@@ -663,7 +671,10 @@
         renderList: () => renderList()
     };
 
+    document.addEventListener('usmt:data', () => { loaded = true; });
     document.addEventListener('DOMContentLoaded', () => {
+        maybeRender(true);
+        setTimeout(() => { if (!loaded) { loaded = true; maybeRender(true); } }, 8000);   // never stuck on the placeholder
         const orig = window.switchPage;
         if (typeof orig === 'function') {
             window.switchPage = function (id) {