Add PWA support — installable as home screen app

Adds Web App Manifest, a minimal Service Worker, and Apple/Android meta
tags so the app can be added to a phone home screen and opens full-screen
in standalone mode (no browser chrome).

- static/manifest.json: name, short_name, display=standalone, icons
- static/sw.js: minimal SW served at /sw.js (root scope) via new Flask route
- static/icon-192.png, icon-512.png: generated by sbin/gen_icons.py (stdlib only)
- base.html: manifest link, theme-color, apple-mobile-web-app-* tags, SW registration

templates/base.html

@@ -4,6 +4,12 @@
   <meta charset="UTF-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <title>{% block title %}Battery Tracker{% endblock %}</title>
+  <link rel="manifest" href="/static/manifest.json">
+  <meta name="theme-color" content="#2563eb">
+  <meta name="apple-mobile-web-app-capable" content="yes">
+  <meta name="apple-mobile-web-app-status-bar-style" content="default">
+  <meta name="apple-mobile-web-app-title" content="Batteries">
+  <link rel="apple-touch-icon" href="/static/icon-192.png">
   <style>
     /* ─── Color variables (light mode) ──────────────────────────────── */
     :root {
@@ -320,5 +326,10 @@
 
     {% block content %}{% endblock %}
   </div>
+  <script>
+    if ('serviceWorker' in navigator) {
+      navigator.serviceWorker.register('/sw.js');
+    }
+  </script>
 </body>
 </html>