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()