ui: persist playground state across route navigation (#525)

- Keep Playground component mounted when navigating away, preserving
streaming/generating state
- Add animated gradient effect on Playground nav link when activity is
in progress

ui-svelte/src/stores/route.ts

@@ -0,0 +1,3 @@
+import { writable } from "svelte/store";
+
+export const currentRoute = writable("/");