Blur the page whenever the window isn't in front

Losing focus -- another tab, another window, another app -- now puts the
whole page behind a blur, so an app switcher's snapshot or a glance over
the shoulder gives nothing away. It comes back on focus, on the tab
becoming visible again, and on the first touch or key, which lands on the
veil rather than on a page nobody could see.

Painted as a backdrop filter on a fixed overlay above everything the app
can show: a filter on the page itself would make every fixed element
position against the wrong box.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MPZFnNdHbPGDTqQUNiE4ZN

frontend/js/main.js

@@ -3,6 +3,10 @@ window.App = window.App || {};
 (function() {
     // App bootstrap: initialize storage, render UI, and load the first page.
     async function initApp() {
+        // Armed before anything is on screen: there is nothing to show behind
+        // a blur yet, but the window can lose focus during startup too.
+        App.privacy.start();
+
         await App.storage.ensureDefaults();
         App.ui.applyTheme();
         App.ui.applyPreferredQuality();