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_head.html

@@ -0,0 +1,6 @@
+<link rel="manifest" href="{{ manifest_href }}">
+<meta name="theme-color" content="#1a1d27">
+<meta name="apple-mobile-web-app-capable" content="yes">
+<meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
+<link rel="apple-touch-icon" href="/static/icon-192.png">
+<script src="/static/pwa.js" defer></script>

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>

templates/index.html

@@ -5,6 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <title>Idle Fantasy Viewer</title>
   <link rel="icon" href="/static/favicon.svg" type="image/svg+xml">
+  {% include '_pwa_head.html' %}
   <link rel="stylesheet" href="/static/style.css">
   {% include '_analytics.html' %}
   <script src="/static/vendor/chart.umd.min.js" defer></script>
@@ -33,6 +34,7 @@
         <button class="nav-btn" data-tab="backup" data-i18n="nav.backup">Backup</button>
       </nav>
       <div class="sidebar-footer">
+        {% include '_pwa_hint.html' %}
         <label class="lang-label" for="locale-select">
           <span data-i18n="settings.language">Language</span>
           <select id="locale-select" class="select-input lang-select">

templates/landing.html

@@ -5,6 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <title>Idle Fantasy Viewer</title>
   <link rel="icon" href="/static/favicon.svg" type="image/svg+xml">
+  {% include '_pwa_head.html' %}
   <link rel="stylesheet" href="/static/style.css">
   {% include '_analytics.html' %}
   <script src="/static/i18n.js" defer></script>
@@ -41,6 +42,8 @@
       <p class="landing-hint" id="create-status" hidden></p>
     </div>
 
+    {% include '_pwa_hint.html' %}
+
     <div class="landing-warning">
       <strong data-i18n="viewer.warningTitle">Important</strong>
       <p data-i18n="viewer.warningBody">