refactor(web): open produced files through the Host, not over HTTP

Scope decision: previews for a browser that is not on the Host machine are
not supported. With that settled, host.openPath answers the supported case
completely — a file:// document in a real browser has full page capabilities
and no reach into /api — and the HTTP serving this branch had built answered
only the unsupported one.

Removed: the /f route and its listener, the workspace-file URL shape,
ApiProxy.workspaceRootOf, ConnectionHandle.fileUrl, and the port published
into the index page.

Kept, and finished:
- the produced-files row a turn ends with, derived from mutation locations;
- the path link now reads as a link at rest, not only on hover — the reported
  "I can't open what it made" was this, sitting on a working capability;
- the Host opener prefers the default BROWSER for .html/.htm/.xhtml/.svg, so
  a developer who binds .html to an editor still gets a rendered page
  (macOS via the LaunchServices https handler, Linux via $BROWSER, every
  failure falling back to the default application).

The retired designs and their measurements stay in the Agent Note, including
why same-origin serving was unsafe and why the sandbox that fixed it broke
the pages invisibly.

tsconfig.host.json

@@ -38,7 +38,7 @@
     "apps/web/tests/access-confirmation.e2e.ts",
     "apps/web/tests/shipped-composition.e2e.ts",
     "apps/web/tests/startup-auto-selection.e2e.ts",
-    "apps/web/tests/workspace-file-open.e2e.ts",
+    "apps/web/tests/produced-files.e2e.ts",
     "apps/cli/tests/**/*.ts",
     "examples/*/src/**/*.ts",
     "examples/*/start.ts",