feat(gui): make desktop setup flow real and testable

Add a GUI-first setup gate and runtime state API so desktop onboarding is safe, iterative, and works with isolated fresh-mode installs. Scaffold and wire the desktop shell/runtime pieces so this branch runs end-to-end without disturbing existing user installs.

web/src/lib/resolve-page-title.ts

@@ -18,6 +18,9 @@ export function resolvePageTitle(
   pluginTabs: { path: string; label: string }[],
 ): string {
   const normalized = pathname.replace(/\/$/, "") || "/";
+  if (normalized === "/setup") {
+    return "Setup";
+  }
   if (normalized === "/") {
     return t.app.nav.sessions;
   }