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/playgroundActivity.ts

@@ -0,0 +1,18 @@
+import { writable, derived } from "svelte/store";
+
+const chatStreaming = writable(false);
+const imageGenerating = writable(false);
+const speechGenerating = writable(false);
+const audioTranscribing = writable(false);
+
+export const playgroundActivity = derived(
+  [chatStreaming, imageGenerating, speechGenerating, audioTranscribing],
+  ([$chat, $image, $speech, $audio]) => $chat || $image || $speech || $audio
+);
+
+export const playgroundStores = {
+  chatStreaming,
+  imageGenerating,
+  speechGenerating,
+  audioTranscribing,
+};

ui-svelte/src/stores/route.ts

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