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/manifest.webmanifest

@@ -0,0 +1,29 @@
+{
+ "name": "SP-404MK2 layers",
+ "short_name": "404 layers",
+ "start_url": "./",
+ "scope": "./",
+ "display": "standalone",
+ "orientation": "portrait",
+ "background_color": "#151719",
+ "theme_color": "#2a2d30",
+ "description": "Combo trainer for the Roland SP-404MK2",
+ "icons": [
+  {
+   "src": "icons/icon-192.png",
+   "sizes": "192x192",
+   "type": "image/png"
+  },
+  {
+   "src": "icons/icon-512.png",
+   "sizes": "512x512",
+   "type": "image/png"
+  },
+  {
+   "src": "icons/maskable-512.png",
+   "sizes": "512x512",
+   "type": "image/png",
+   "purpose": "maskable"
+  }
+ ]
+}