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.

apps/solidcraft/src/main.rs

@@ -157,6 +157,8 @@ fn services(cc: &eframe::CreationContext<'_>) -> Services {
         graphics_path: graphics_path().map(|p| p.to_string_lossy().to_string()),
         http_get: Some(std::sync::Arc::new(http_get)),
         http_download: Some(std::sync::Arc::new(http_download)),
+        // Already an installed app: only the browser build offers to install.
+        install_app: None,
         show_in_folder: Some(Box::new(|path| {
             let folder = std::path::Path::new(path).parent().unwrap_or_else(|| std::path::Path::new("."));
             let mut command = std::process::Command::new(if cfg!(target_os = "windows") {