feat: list files in the extras picker, not just folders

The extras control accepts a file — the field and its label said so — and the
listing under it could not show one, because the Host primitive behind the picker
browses folders and nothing else. So a grant for a single file meant typing its
name exactly, which is the one thing a listing exists to avoid.

The Host has a second lister and it does return files: `remote.workspaceFiles.list`,
the same listing behind the sidebar's own file tree, confined to the session's
workspace. The extras picker now takes that one while the level is inside the
session folder — which is where an extra usually is — and falls back to the folder
lister above it, saying so in the dialog rather than letting file rows vanish. The
focus picker never offers files: a focus is a folder, so a file row there would
offer something that cannot be chosen.

The two listings normalise into one level shape, so the picker does not care which
answered, and a row now carries its type: a folder is a way down, a file is a
thing to take. Clicking a file fills the path field and marks the row, and the
footer adds it in declared form. Pressing Enter on a name the level already lists
is no longer reported as a failure — the path is exactly what the dialog is for.
Everything stays additive: with no workspace file service, or above the session
folder, the picker lists folders exactly as before.

package.json

@@ -51,7 +51,8 @@
         "@deepseek-ai/dsh-client-ui-settings",
         "@deepseek-ai/dsh-client-ui-plugin-manager",
         "@deepseek-ai/dsh-client-ui-conversation",
-        "@deepseek-ai/dsh-client-ui-workspace"
+        "@deepseek-ai/dsh-client-ui-workspace",
+        "@deepseek-ai/dsh-api-workspace-files"
       ]
     }
   },