Add the browser send proxy and web sender

crates-server/yaak-send-proxy: a stateless executor over yaak-http's
HttpTransaction. It takes a rendered request, streams timeline events,
the response head, body chunks and the resulting cookies back as NDJSON,
and keeps nothing. Private/loopback/link-local/metadata ranges are refused
after DNS on every hop (an AddressFilter on the resolver plus a per-hop URL
check), with size caps, a timeout ceiling, a rate limit, host allow/deny
lists and an optional token.

The web host now sends through it: the wasm worker resolves and renders
the request (render_http_request moved into yaak-models so it builds for
wasm; re-exported from its old paths), the tab posts it, and stores what
comes back where the desktop stores it. Requests needing auth plugins or
template functions are refused with the reason until plugins run in the
browser.

crates/yaak-web/pkg/yaak_web.d.ts

@@ -25,6 +25,17 @@ export function blob_put(id: string, bytes: Uint8Array): void;
  */
 export function boot(): Promise<void>;
 
+/**
+ * Resolve and render a request for sending, exactly as the desktop does before it puts the
+ * request on the network: the environment chain, inherited headers and auth, request
+ * settings, the cookie jar. Nothing here touches a socket. What comes back is what the tab
+ * posts to the send proxy.
+ *
+ * Refuses, with a message the user can act on, when the request needs something this host
+ * doesn't have: an authentication plugin, or a template function.
+ */
+export function prepare_http_send(payload: any): Promise<any>;
+
 /**
  * Run one command as `label` (the calling tab's identity, which stands in for
  * the desktop's window label on every write it makes).