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/components/playground/AudioInterface.svelte

@@ -2,6 +2,7 @@
   import { models } from "../../stores/api";
   import { persistentStore } from "../../stores/persistent";
   import { transcribeAudio } from "../../lib/audioApi";
+  import { playgroundStores } from "../../stores/playgroundActivity";
   import ModelSelector from "./ModelSelector.svelte";
 
   const selectedModelStore = persistentStore<string>("playground-audio-model", "");
@@ -22,6 +23,10 @@
 
   let canTranscribe = $derived(selectedFile !== null && $selectedModelStore !== "" && !isTranscribing);
 
+  $effect(() => {
+    playgroundStores.audioTranscribing.set(isTranscribing);
+  });
+
   function validateFile(file: File): { valid: boolean; error?: string } {
     const ext = '.' + file.name.split('.').pop()?.toLowerCase();