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.

lib/drawio/drawio-config.ts

@@ -47,7 +47,10 @@ export function getDrawioSrc(source: DrawioSource, dark: boolean): string {
         proto: "json",
         configure: "1",
     }
+    // Without a fixed theme, embedded draw.io shows Extras > Theme only
+    // with themes=1 (its isThemeMenuVisible)
     if (source.ui) params.ui = source.ui
+    else params.themes = "1"
     Object.assign(params, {
         spin: "0",
         libraries: "0",