Phase 1: wrap the reference web app in Capacitor for Android

Wraps reference/sp404-layers as a sideloadable APK without editing the
reference, which stays the frozen spec for the Phase 2 React port.

- sync-www.sh builds www/ from reference/ + native/, asserting each patch
  anchor so a reference change fails loudly rather than shipping broken
- drops the service worker (would serve a stale app after an APK update)
- haptics via a navigator.vibrate override, so buzz() needs no change
- localStorage mirrored into Preferences; sync load()/save() untouched
- hardware back: sheet -> Explore tab -> exit
- edge-to-edge handled in MainActivity: Android 16 + targetSdk 36 removes
  the opt-out, and a WebView never fills env(safe-area-inset-*), so insets
  are pushed into CSS custom properties instead

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

reference/sw.js

@@ -0,0 +1,14 @@
+const V="404layers-v2";
+const FILES=["./", "index.html", "manifest.webmanifest", "icons/icon-192.png", "icons/icon-512.png", "icons/maskable-512.png", "fonts/barlow-condensed-600.woff2", "fonts/barlow-400.woff2", "fonts/barlow-condensed-700.woff2", "fonts/barlow-500.woff2", "fonts/barlow-condensed-500.woff2", "fonts/barlow-600.woff2"];
+self.addEventListener("install",e=>e.waitUntil(caches.open(V).then(c=>c.addAll(FILES)).then(()=>self.skipWaiting())));
+self.addEventListener("activate",e=>e.waitUntil(caches.keys().then(ks=>Promise.all(ks.filter(k=>k!==V).map(k=>caches.delete(k)))).then(()=>self.clients.claim())));
+self.addEventListener("fetch",e=>{
+  if(e.request.method!=="GET")return;
+  const url=new URL(e.request.url);if(url.origin!==location.origin)return;
+  // network first for the page (so updates land), cache first for everything else
+  if(e.request.mode==="navigate"){
+    e.respondWith(fetch(e.request).then(r=>{const c=r.clone();caches.open(V).then(x=>x.put("index.html",c));return r}).catch(()=>caches.match("index.html")));
+    return;
+  }
+  e.respondWith(caches.match(e.request).then(r=>r||fetch(e.request)));
+});