Installable web app (PWA) with offline use #13

Make https://nmsii.dlu.one installable and fully usable offline, for a phone or second screen next to the game.

• Web app manifest, icons (incl. maskable) and shortcuts to Recipes, Nanites, Cooking, Portals
• Service worker: app shell cached, game data refreshed in the background so a patch's numbers arrive on the next visit
• "Install app" button where the browser allows it, an "Add to Home Screen" hint on iOS
• A reload prompt when a new version is ready (never swapped mid-session), and an offline marker

Status: in progress.