ui: finish shadcn migration and remove legacy shim

Convert the remaining .btn usages (Concurrency, Performance, CaptureDialog)
to shadcn Button, fix CaptureDialog/PerformanceChart styles to shadcn
tokens, and remove the transitional legacy palette aliases and component
classes from index.css. Drop the now-unused lucide-svelte and shadcn-svelte
dependencies.

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

ui-svelte/src/components/PerformanceChart.svelte

@@ -143,6 +143,6 @@
   });
 </script>
 
-<div class="card p-4 h-[300px]">
+<div class="bg-card text-card-foreground h-[300px] rounded-xl border p-4 shadow-sm">
   <canvas bind:this={canvas}></canvas>
 </div>