new_app: Create or stop a local web application

Description

Create a local web application based on R's internal httpd.

new_app(name, handler, open = interactive(), host = "127.0.0.1", port = NULL)
stop_app(name = names(.proxy$apps))

new_app() returns the app URL invisibly. stop_app() returns nothing.

name: App name (a character string). Use name = '' to serve the app at the root URL http://host:PORT/ via the proxy. If name is non-empty, the app is served directly by R's internal httpd at /custom/name/. Each name must be unique; calling new_app() with an existing name replaces that app. For stop_app(), a character vector of app names to stop; defaults to all running apps.
handler: A function with signature function(path, query, post, headers) that handles HTTP requests and returns a response list.
open: Whether to open the app URL in a browser, or a function to open it. In non-interactive sessions this also controls whether the call blocks: passing open = FALSE explicitly suppresses both browser-opening and blocking.
host: Bind address for the proxy ("127.0.0.1" or "0.0.0.0"). Only used when name = ''.
port: Candidate proxy ports when name = ''. The first available port is selected (falling back to random_port() if needed). Ignored when name != ''.

new_app() has two modes:

name = '': start an app behind a lightweight proxy at http://host:PORT/, forwarding to R's internal httpd path /custom/xfun:PORT/.
name != '': register a legacy app directly on R's internal httpd at http://127.0.0.1:BACKEND/custom/name/ (compatible with older versions).

stop_app() deregisters one or more running apps.