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/ImageInterface.svelte

@@ -2,6 +2,7 @@
   import { models } from "../../stores/api";
   import { persistentStore } from "../../stores/persistent";
   import { generateImage } from "../../lib/imageApi";
+  import { playgroundStores } from "../../stores/playgroundActivity";
   import ModelSelector from "./ModelSelector.svelte";
   import ExpandableTextarea from "./ExpandableTextarea.svelte";
 
@@ -17,6 +18,10 @@
 
   let hasModels = $derived($models.some((m) => !m.unlisted));
 
+  $effect(() => {
+    playgroundStores.imageGenerating.set(isGenerating);
+  });
+
   async function generate() {
     const trimmedPrompt = prompt.trim();
     if (!trimmedPrompt || !$selectedModelStore || isGenerating) return;