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

static/manifest.json

@@ -0,0 +1,12 @@
+{
+  "name": "Battery Tracker",
+  "short_name": "Batteries",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#ffffff",
+  "theme_color": "#2563eb",
+  "icons": [
+    { "src": "/static/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/static/icon-512.png", "sizes": "512x512", "type": "image/png" }
+  ]
+}