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.

apps/gui/src-tauri/build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    tauri_build::build();
+}