Demo popup: link the source repo, keep the buttons pinned on screen

The popup text now scrolls inside the card while the buttons stay at the
bottom, so phone visitors see 'Explore the dashboard' right away (it goes
first and full width on narrow screens). The card height uses dvh so the
top edge isn't cut off behind a phone's address bar.

web/demo/demo.js

@@ -937,7 +937,9 @@
             modal.className = 'usmt-demo-modal';
             modal.innerHTML = `
                 <div class="usmt-demo-card" role="dialog" aria-labelledby="usmt-demo-title">
+                  <div class="usmt-demo-body">
                     <h2 id="usmt-demo-title">You're looking at a live demo</h2>
+                    <p class="usmt-demo-src">Source code: <a href="https://gitea.yoyos.sh/yoyos/usmt-dash" target="_blank" rel="noopener">gitea.yoyos.sh/yoyos/usmt-dash</a></p>
                     <p>This is the real dashboard, served by the real <code>USMT-Server.ps1</code> HttpListener and fed by the real
                        <code>USMT-Worker.ps1</code> polling loop, running in PowerShell 7 in a container. The only difference is the data
                        source: instead of SCCM, Active Directory and SMB, the worker talks to a simulated fleet of 200 devices whose
@@ -950,6 +952,7 @@
                        fleet &middot; add a device such as <code>PRVDT-7QK4M2XA</code> under <em>Tracked Devices</em> &middot;
                        click a device for its details &middot; <em>All Devices</em> shows apps missing from replacement machines &middot;
                        edit an article under <em>Documentation</em>.</p>
+                  </div>
                     <div class="usmt-demo-actions">
                         <button type="button" class="usmt-demo-reset">Reset my changes</button>
                         <button type="button" class="usmt-demo-terms">Open terminals</button>