fix(mcp-server): review fixes for the preview URL and the theme menu

The stalled-tab note names the page start_session opened (the shell
with DRAWIO_PREVIEW_UI=shell) instead of the classic page. With the
shell not built, start_session opens the classic page and the server
says so at startup, as it does for a missing dist/drawio. BROWSER=none
skips the system browser, as Vite and CRA dev servers do: the e2e tests
set it. Without a fixed DRAWIO_UI, the shell's draw.io URL carries
themes=1 so Extras > Theme is offered, as on the classic page.

packages/mcp-server/tests/http-server.test.ts

@@ -870,6 +870,15 @@ describe("canvas shell page", () => {
         expect(previewUrl(6002, "mcp-x", "classic")).toBe(
             "http://localhost:6002?mcp=mcp-x",
         )
+        // Not built: the classic page, which works, rather than a 404
+        setShellDir(null)
+        try {
+            expect(previewUrl(6002, "mcp-x", "shell")).toBe(
+                "http://localhost:6002?mcp=mcp-x",
+            )
+        } finally {
+            setShellDir(shellDir)
+        }
     })
 })