Web: installable as an app, with Install in the File menu (#1233)

The web build ships a manifest so the browser can install it as an app, and offers Install in the File menu while the browser allows it.

assets/README.md

@@ -75,7 +75,7 @@ The CLI still supports arbitrary PNG/ICO exports with `solidcraft-cli icon`.
 | Linux, Nix, deb/rpm, tar, Flatpak, Snap | Copy canonical hicolor images; icon name stays `ai.storyteller.solidcraft` |
 | AppImage | Uses the Linux 256 px image and `.DirIcon` alias |
 | FreeBSD | Shares Linux hicolor images and freedesktop metadata |
-| Web | HTML/Trunk stays in `apps/solidcraft-web/`; copies Japanese font/licence to `fonts/` and uses the collected CSS; no favicon/PWA icon currently exists |
+| Web | HTML/Trunk and `manifest.webmanifest` stay in `apps/solidcraft-web/`; copies Japanese font/licence to `fonts/`, uses the collected CSS, and copies the Linux 32/256/512 px images to `icons/` for the favicon and installed app |
 | Docs and link previews | `xtask book` copies the shared card to the existing `/solidcraft/og.png` public path |
 
 ## Code and provenance