ui: migrate Rerank and normalize remaining views to shadcn tokens

- RerankInterface uses Button/Input/Textarea/ToggleGroup
- normalize legacy color utilities and lucide imports across the
  remaining playground interfaces, Performance and CaptureDialog

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UmuGqwNBJNEAMaWsdCDqUC

ui-svelte/src/components/playground/PlaceholderTab.svelte

@@ -7,7 +7,7 @@
 </script>
 
 <div class="flex items-center justify-center h-full">
-  <div class="text-center text-txtsecondary">
+  <div class="text-muted-foreground text-center">
     <p class="text-lg">{featureName}</p>
     <p class="text-sm mt-2">To be implemented</p>
   </div>