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/package.json

@@ -22,7 +22,6 @@
     "bits-ui": "^2.18.1",
     "clsx": "^2.1.1",
     "paneforge": "^1.0.2",
-    "shadcn-svelte": "^1.3.0",
     "svelte": "^5.46.4",
     "svelte-check": "^4.1.4",
     "tailwind-merge": "^3.6.0",
@@ -38,7 +37,6 @@
     "chart.js": "4.5.1",
     "highlight.js": "^11.11.1",
     "katex": "^0.16.28",
-    "lucide-svelte": "^0.563.0",
     "rehype-katex": "^7.0.1",
     "rehype-stringify": "^10.0.1",
     "remark-gfm": "^4.0.1",