Add x-app-url header to tunnel requests

docs/GUIDE.md

@@ -686,16 +686,16 @@ toes share my-app
 
 **`toes unshare [name]`** — Stop sharing an app.
 
-When an app is shared, every proxied request includes an `x-app-url` header set to the app's public tunnel URL (e.g., `https://myapp.toes.space`). When not shared, it's the local URL (e.g., `http://myapp.toes.local`).
+Every request to your app includes an `x-app-url` header with the app's public-facing URL. When shared, this is the tunnel URL (e.g., `https://myapp.toes.space`). When not shared, it's the local URL (e.g., `http://myapp.toes.local`). This works whether the request arrives through the local proxy or through a tunnel.
 
-Use `appUrl()` from `@because/toes` to read this header:
+Use `appUrl()` from `@because/toes` to read it — never hardcode your app's URL:
 
 ```tsx
 import { appUrl } from '@because/toes'
 
 app.get('/callback', c => {
   const url = appUrl(c.req.raw)
-  // url = "https://myapp.toes.space" if shared, "http://myapp.toes.local" otherwise
+  // "https://myapp.toes.space" when shared, "http://myapp.toes.local" otherwise
   return c.redirect(`${url}/done`)
 })
 ```

src/server/tunnels.ts

@@ -177,6 +177,11 @@ function openTunnel(appName: string, port: number, subdomain?: string, isReconne
     subdomain,
     reconnect: false,
 
+    onRequest(req) {
+      const app = getApp(appName)
+      if (app?.tunnelUrl) req.headers['x-app-url'] = app.tunnelUrl
+    },
+
     onOpen(assignedSubdomain) {
       hostLog(`Tunnel open: ${appName} -> ${assignedSubdomain}`)