feat(web): installable PWA shell so the web build works as an iPhone home-screen app

There is no iOS binary — CI only produces a signed APK — so Safari's Add to
Home Screen is the only way onto an iPhone today. Without a manifest and the
apple-* meta tags that lands a bookmark that opens inside browser chrome, so
add both, plus a viewport that covers the notch and stops focus-zoom.

app/public/manifest.json

@@ -0,0 +1,16 @@
+{
+  "name": "GTD",
+  "short_name": "GTD",
+  "description": "Capture, clarify, do.",
+  "start_url": "/today",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "portrait",
+  "background_color": "#F7F7F9",
+  "theme_color": "#F7F7F9",
+  "icons": [
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
+  ]
+}