Add PWA manifest, service worker, and install hint.

Enables home-screen installation with per-viewer scope and platform-specific guidance in EN/DE.

Co-authored-by: Cursor <cursoragent@cursor.com>

templates/_pwa_hint.html

@@ -0,0 +1,10 @@
+<div id="pwa-install-hint" class="pwa-hint" hidden>
+  <div class="pwa-hint-text">
+    <strong data-i18n="pwa.hintTitle">Install as app</strong>
+    <p id="pwa-hint-body" data-i18n="pwa.hintBody">Add this viewer to your home screen for quick access.</p>
+  </div>
+  <div class="pwa-hint-actions">
+    <button type="button" id="pwa-install-btn" class="viewer-copy-btn" hidden data-i18n="pwa.install">Install</button>
+    <button type="button" id="pwa-hint-dismiss" class="import-report-dismiss" data-i18n-title="actions.dismiss" title="Dismiss">×</button>
+  </div>
+</div>