Docs: add canvas host usage

docs/mac/browser.md

@@ -42,6 +42,7 @@ Clawdis already uses:
 For the clawd browser-control server, use “family” ports:
 - Browser control HTTP API: `18791` (bridge + 1)
 - Browser CDP/debugging port: `18792` (control + 1)
+- Canvas host HTTP (optional): `18793` (next free port; see `docs/configuration.md`)
 
 The user usually only configures the **control URL** (port `18791`). CDP is an internal detail.