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/capabilities/default.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "../gen/schemas/desktop-schema.json",
+  "identifier": "default",
+  "description": "enables the default permissions",
+  "windows": [
+    "main"
+  ],
+  "permissions": [
+    "core:default"
+  ]
+}