refactor: convert SPA to multi-page architecture

- Add scene_onboarding.html, scene_dashboard.html, scene_settings.html
- Add onboarding.js, dashboard.js, settings.js scripts
- Update vite.config.js for multi-page build
- Navigation via window.location.href between pages
- Bottom nav bar on dashboard only

src-tauri/build.rs

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