Add /display manifest for Android home-screen install

Android Chrome only reads manifest at install time from current page. Root
manifest has start_url '.' (root). Installing from /display launched root.

Fix: separate display-manifest.json (start_url /display, scope /, landscape).
App.js swaps link[rel=manifest] href when on /display path. Install from
/display now launches /display standalone.

public/display-manifest.json

@@ -0,0 +1,27 @@
+{
+  "short_name": "TTRPG Display",
+  "name": "TTRPG Initiative Tracker — Player Display",
+  "icons": [
+    {
+      "src": "favicon.ico",
+      "sizes": "64x64 32x32 24x24 16x16",
+      "type": "image/x-icon"
+    },
+    {
+      "src": "logo192.png",
+      "type": "image/png",
+      "sizes": "192x192"
+    },
+    {
+      "src": "logo512.png",
+      "type": "image/png",
+      "sizes": "512x512"
+    }
+  ],
+  "start_url": "/display",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "landscape",
+  "theme_color": "#1A202C",
+  "background_color": "#1A202C"
+}