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.

packages/client/ui-conversation/src/client/apply.ts

@@ -2,7 +2,6 @@
 import type { Context } from 'cordis'
 import { resolveSlotLabel, type BoundActions } from '@deepseek-ai/dsh-client-ui-slots'
 import type { ISessions, SessionId } from '@deepseek-ai/dsh-client-runtime/client'
-import type { ConnectionHandle } from '@deepseek-ai/dsh-client-connection/client'
 import type {} from '@deepseek-ai/dsh-client-ui-layout/client'
 // Type-only: pulls the locale plugin's Context merge (ctx.locale).
 import type {} from '@deepseek-ai/dsh-client-locale/client'
@@ -43,7 +42,7 @@ declare module '@deepseek-ai/dsh-client-ui-slots' {
 }
 
 /** Services required by the conversation plugin. */
-export const inject = ['slots', 'layout', 'sessions', 'workspaces', 'locale', 'connection']
+export const inject = ['slots', 'layout', 'sessions', 'workspaces', 'locale']
 
 // Static no-session sources for the composer-bar hooks compartment: module
 // constants so the render side's per-source hook cache (observableHook) keeps
@@ -276,16 +275,6 @@ export function apply(ctx: Context): void {
         },
         openFile: (path) => {
           const cwd = sessions.list.getSnapshot().byId[sessionId]?.cwd
-          // A file inside the workspace opens in a new tab on the transport's
-          // workspace-file origin, so a browser that is not on the Host machine
-          // can still see what the agent produced. Anything outside it has no
-          // served URL and falls back to the Host's own opener, which is
-          // loopback-only by the /api trust fence.
-          const url = (ctx.get('connection') as ConnectionHandle).fileUrl(sessionId, cwd, path)
-          if (url !== undefined) {
-            window.open(url, '_blank', 'noopener,noreferrer')
-            return
-          }
           void workspaces.openPath(resolveToolPath(cwd, path)).catch(() => {
             // Host/OS open failures stay silent in the chat row; the native
             // app surfaces its own error dialog when the path is unusable.

packages/client/ui-conversation/src/client/chat/ToolRow.module.css

@@ -84,7 +84,10 @@
   color: var(--dsw-alias-label-tertiary);
 }
 
-/* File-tool path: same geometry as .summary; hover underline + pointer. */
+/* File-tool path: same geometry as .summary, but it must READ as a link. A
+   path styled exactly like the surrounding prose, underlined only on hover, is
+   an affordance nobody finds — the reported "I can't open what it made" was
+   this, not a missing capability. */
 .fileLink {
   flex: 1 1 auto;
   min-width: 0;
@@ -99,12 +102,16 @@
   text-align: left;
   font-size: 14px;
   line-height: 24px;
-  color: var(--dsw-alias-label-tertiary);
+  color: var(--dsw-alias-label-secondary);
+  text-decoration: underline;
+  text-decoration-color: var(--dsw-alias-label-quaternary);
+  text-underline-offset: 3px;
   cursor: pointer;
 }
 
 .fileLink:hover {
-  text-decoration: underline;
+  color: var(--dsw-alias-label-primary);
+  text-decoration-color: currentColor;
 }
 
 /* Error row's collapsed summary: the failure's first line in the error color. */