Replace Memo demo with a URL-payload WebApp Shell

README.md

@@ -0,0 +1,77 @@
+# WebApp Shell
+
+A tiny, offline-first **PWA shell** that renders self-contained HTML pages encoded
+directly into the URL. It's a successor to opening `data:text/html;base64,…` URLs
+from iOS Shortcuts — but on a real, persistent origin.
+
+**Live:** https://appshr.github.io/webapp-shell/
+
+## What it does
+
+1. Open the shell. Paste a self-contained HTML page into the builder.
+2. It encodes your HTML (base64url, optionally gzip-compressed) into a link like:
+   ```
+   https://appshr.github.io/webapp-shell/#app=<base64url>
+   https://appshr.github.io/webapp-shell/#appz=<gzip+base64url>
+   ```
+3. Opening that link renders your page full-screen inside the shell.
+
+## Why it beats raw `data:` URLs
+
+| | `data:` URL | WebApp Shell |
+|---|---|---|
+| Runs offline | yes | yes (service worker) |
+| Persistent `localStorage` / `IndexedDB` | ❌ opaque origin | ✅ real origin, survives launches |
+| Add to Home Screen (full-screen) | ❌ | ✅ |
+| Stable, shareable URL | ❌ | ✅ |
+
+## The payload lives in the URL **hash** (`#`), on purpose
+
+- The hash is **never sent to GitHub's servers**, so there are no URL-length or
+  routing limits, and the service worker always matches the cached shell —
+  meaning **any generated link works fully offline** after the first load.
+- Encoding is **base64url** (`-`/`_`, no padding) so it's URL-safe everywhere.
+- With gzip (Safari 16.4+) the link stays much shorter.
+
+## Using it from Shortcuts
+
+1. Generate a link in the builder and **Copy** it.
+2. Add an **“Open URLs”** action in Shortcuts and paste the link.
+3. Running the shortcut opens your app in Safari — offline, with its data intact.
+
+> Shortcuts opens URLs in Safari (with its minimal toolbar). iOS does **not**
+> allow Shortcuts to launch directly into a chrome-free standalone window — that's
+> an OS limitation, not something a web page can change.
+
+## True full-screen (no Safari chrome)
+
+1. Open your app link, then in Safari tap **Share → Add to Home Screen → Add**.
+2. The icon launches your app full-screen, offline, with its own storage.
+
+The shell auto-injects `apple-mobile-web-app-capable` and a `viewport` meta if your
+page is missing them, so any pasted page becomes full-screen-capable.
+
+## Why not `webapp://`?
+
+Custom URL schemes can only be claimed by **native** apps on iOS (via Info.plist).
+A web page / PWA cannot register `webapp://`, so it can never be launched that way.
+The `https://` links this tool generates do everything that scheme was meant to do.
+
+## Offline guarantee
+
+The first online load caches the shell permanently via a service worker. After that
+the shell and every link work with no network. Your app only needs the internet if
+**its own code** fetches something external — keep pages self-contained to stay 100% offline.
+
+## ⚠️ Security note
+
+Anything you load runs on the `appshr.github.io` origin with full access to its
+storage. Only open links you created or trust: a malicious page could read or wipe
+the data of your other shell apps.
+
+## Files
+
+- `index.html` — the shell (renderer + builder UI)
+- `sw.js` — service worker (offline cache)
+- `manifest.webmanifest` — PWA manifest
+- `icon.svg`, `icon-192.png`, `icon-512.png` — icons