Adds manifest.json + a 512x512 icon and links them from layout.njk, so
Edge/Chrome can install the site as a standalone app window from a
desktop shortcut while it keeps loading live from the server.
nav had no flex-wrap, so the brand + 3 links overflowed a phone-width
viewport at the site's 150% base font size - Customers (last link) got
clipped off-screen, only reachable by rotating to landscape. Nav now
wraps, and below 430px the brand drops to its own line above the links.
eric