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/sw.js

@@ -0,0 +1,5 @@
+// Minimal service worker — required for PWA installability
+self.addEventListener('install', function(e) { self.skipWaiting(); });
+self.addEventListener('activate', function(e) { e.waitUntil(clients.claim()); });
+// No caching — app requires live server data
+self.addEventListener('fetch', function(e) {});