feat(web): serve workspace files from their own origin

A sandbox header bought isolation by taking the document's origin away, and
measuring that cost decided against it: the reported artifact throws
SecurityError on load, and because an uncaught exception aborts the rest of
its <script>, every listener declared after that line — theme toggle, mobile
menu, model tabs — never binds. Two of the four artifacts in the reporting
user's workspace were dead pages under it, and they still looked right.

A second listener on the API's host, answering /f and nothing else, is the
same boundary without the amputation: cross-origin to /api (refused by the
Origin fence and by CORS), same-origin with itself (localStorage, cookies and
fetch all work). Its port is published into the index page; the browser half
reads it to address previews, and its absence — the keyless fixture lane — is
what makes a file row fall back to the Host opener instead of a dead tab.

fileUrl moves from IWorkspaces to ConnectionHandle: the transport owns both
the listener that serves the bytes and the port that addresses it.

docs/config-catalog.md

@@ -296,7 +296,7 @@ export interface ConnectionConfig {
 }
 ```
 
-Source: [`packages/client/connection/src/index.ts:26`](../packages/client/connection/src/index.ts)
+Source: [`packages/client/connection/src/index.ts:25`](../packages/client/connection/src/index.ts)
 
 ## `@deepseek-ai/dsh-client-hmr`