Save QR image stranded the window on a picture of itself

Build 124. Reported from the booth: the button did not download anything and
there was no way back off the QR.

Both halves are the same fault. The window is served from the asset port and
the code comes off the congregation port, so the link the button built was
CROSS-ORIGIN — and a cross-origin <a download> has its download attribute
ignored and NAVIGATES instead. So the file never saved, and the window went to
the image. The app has no back button, no reload item and no address bar, so
that is a one-way trip: a headless booth Mac showing a full-screen QR with the
operator locked out of the desk. It fetches the code and downloads a blob now,
which is same-origin and actually saves.

The second change is the one that matters more. Nothing was stopping the app
window being replaced by any navigation at all, and the only reason this had
never bitten before is that nothing had tried. will-navigate now refuses to
replace the app page, logs what it refused, and sends anything external to the
browser. Our own pages are unaffected — the operator console and the poster
open as their own windows through setWindowOpenHandler, which is a different
path entirely.

Found because the build number went to the wrong field: buildNumber is the
source of truth and build.buildVersion is synced from it by predist, which
caught it and rebuilt correctly.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/main.js

@@ -662,6 +662,27 @@ function createWindow() {
              : '/index.html';
   win.loadURL(assetOrigin + page);
 
+  /*
+   * NOTHING replaces the app in this window. There is no back button, no
+   * reload item and no address bar, so a window that leaves the app page is a
+   * window the operator cannot get back from — which is precisely how build
+   * 123 stranded a booth Mac on a full-screen QR image. Our own pages still
+   * open as their OWN windows through setWindowOpenHandler below; this only
+   * refuses to replace the app itself, and sends anything external to the
+   * browser where it belongs.
+   */
+  win.webContents.on('will-navigate', (e, url) => {
+    if (url === assetOrigin + page) return;     /* reloading ourselves is fine */
+    e.preventDefault();
+    console.error('[main] refused to navigate the app window to ' + url);
+    let local = false;
+    try {
+      const u = new URL(url);
+      local = (u.hostname === '127.0.0.1' || u.hostname === 'localhost');
+    } catch (_) { local = false; }
+    if (!local) shell.openExternal(url).catch(() => {});
+  });
+
   win.webContents.on('did-finish-load', () => console.log('[main] page loaded'));
   win.webContents.on('did-fail-load', (_e, code, desc, url) =>
     console.error('[main] page FAILED to load: ' + code + ' ' + desc + ' ' + url));