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" }
+  ]
+}

app/src/app/+html.tsx

@@ -0,0 +1,49 @@
+// The HTML shell for the web build.
+//
+// The web export is the iPhone story — there is no iOS binary, so Safari's "Add
+// to Home Screen" has to produce something that opens full-screen and looks
+// installed rather than a bookmark with a browser chrome on top. That takes the
+// manifest, the apple-* meta tags and a viewport that covers the notch; none of
+// them are reachable from app.json, so the shell is written out by hand.
+import { ScrollViewStyleReset } from "expo-router/html";
+import type { PropsWithChildren } from "react";
+
+// Match the app background on both schemes so the status bar and the rubber-band
+// overscroll never flash white against a dark list.
+const background = `
+:root { color-scheme: light dark; }
+body { background-color: #F7F7F9; overscroll-behavior-y: none; }
+@media (prefers-color-scheme: dark) { body { background-color: #0C0C0F; } }
+`;
+
+export default function Root({ children }: PropsWithChildren) {
+  return (
+    <html lang="en">
+      <head>
+        <meta charSet="utf-8" />
+        <meta httpEquiv="X-UA-Compatible" content="IE=edge" />
+        {/* viewport-fit=cover lets the layout run under the notch; disabling
+            user scaling stops iOS zooming in every time a title field focuses. */}
+        <meta
+          name="viewport"
+          content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no, viewport-fit=cover"
+        />
+
+        <link rel="manifest" href="/manifest.json" />
+        <meta name="application-name" content="GTD" />
+        <meta name="apple-mobile-web-app-capable" content="yes" />
+        <meta name="mobile-web-app-capable" content="yes" />
+        <meta name="apple-mobile-web-app-title" content="GTD" />
+        {/* "default" keeps the iOS status bar legible over a light header. */}
+        <meta name="apple-mobile-web-app-status-bar-style" content="default" />
+        <link rel="apple-touch-icon" sizes="180x180" href="/icon-180.png" />
+        <meta name="theme-color" media="(prefers-color-scheme: light)" content="#F7F7F9" />
+        <meta name="theme-color" media="(prefers-color-scheme: dark)" content="#0C0C0F" />
+
+        <ScrollViewStyleReset />
+        <style dangerouslySetInnerHTML={{ __html: background }} />
+      </head>
+      <body>{children}</body>
+    </html>
+  );
+}