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

app.py

@@ -1,4 +1,4 @@
-from flask import Flask, render_template, redirect, url_for, request, flash, abort
+from flask import Flask, render_template, redirect, url_for, request, flash, abort, send_from_directory
 from sqlalchemy import create_engine, func
 from sqlalchemy.orm import scoped_session, sessionmaker
 
@@ -25,6 +25,11 @@ def create_app(config_object="config"):
     # Dashboard
     # ------------------------------------------------------------------ #
 
+    @app.route("/sw.js")
+    def service_worker():
+        return send_from_directory(app.static_folder, "sw.js",
+                                   mimetype="application/javascript")
+
     @app.route("/")
     def dashboard():
         batteries = db.query(Battery).order_by(Battery.label).all()

sbin/gen_icons.py

@@ -0,0 +1,31 @@
+"""Generate solid-color PNG icons for PWA manifest using stdlib only (no Pillow)."""
+import zlib, struct, os
+
+def make_png(size, rgb=(0x25, 0x63, 0xEB)):
+    """Create a minimal valid RGB PNG of the given size filled with one color."""
+    w = h = size
+    raw = b''
+    for _ in range(h):
+        raw += b'\x00' + bytes(rgb) * w  # filter byte 0 (None) + RGB pixels
+
+    compressed = zlib.compress(raw, 9)
+
+    def chunk(name, data):
+        c = name + data
+        return struct.pack('>I', len(data)) + c + struct.pack('>I', zlib.crc32(c) & 0xffffffff)
+
+    sig  = b'\x89PNG\r\n\x1a\n'
+    ihdr = chunk(b'IHDR', struct.pack('>IIBBBBB', w, h, 8, 2, 0, 0, 0))
+    idat = chunk(b'IDAT', compressed)
+    iend = chunk(b'IEND', b'')
+    return sig + ihdr + idat + iend
+
+
+if __name__ == '__main__':
+    out_dir = os.path.join(os.path.dirname(__file__), '..', 'static')
+    os.makedirs(out_dir, exist_ok=True)
+    for size in [192, 512]:
+        path = os.path.join(out_dir, f'icon-{size}.png')
+        with open(path, 'wb') as f:
+            f.write(make_png(size))
+        print(f'Wrote {path}')

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" }
+  ]
+}

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) {});

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>