ui: add shadcn-svelte foundation and theming

Set up shadcn-svelte components and adopt its design-token system as the
base for modernizing the UI. Switch dark mode from the data-theme attribute
to the .dark class so shadcn primitives theme correctly.

- add components.json, $lib alias (tsconfig + vite), cn() util
- install shadcn primitives under src/lib/components/ui
- rewrite index.css with shadcn tokens (zinc + brand teal accent)
- keep legacy utility/class aliases as a transitional shim
- toggle .dark class from theme store in App.svelte

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UmuGqwNBJNEAMaWsdCDqUC

ui-svelte/package.json

@@ -12,20 +12,30 @@
     "test:watch": "vitest"
   },
   "devDependencies": {
+    "@internationalized/date": "^3.12.2",
+    "@lucide/svelte": "^1.21.0",
     "@sveltejs/vite-plugin-svelte": "^7.0.0",
     "@tailwindcss/vite": "^4.1.8",
     "@tsconfig/svelte": "^5.0.4",
     "@types/hast": "^3.0.4",
     "@types/node": "^25.1.0",
+    "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",
+    "tailwind-variants": "^3.2.2",
     "tailwindcss": "^4.1.8",
+    "tw-animate-css": "^1.4.0",
     "typescript": "~5.8.3",
     "vite": "^8.0.0",
     "vite-plugin-compression2": "^2.5.1",
     "vitest": "^4.1.0"
   },
   "dependencies": {
+    "chart.js": "4.5.1",
     "highlight.js": "^11.11.1",
     "katex": "^0.16.28",
     "lucide-svelte": "^0.563.0",
@@ -35,7 +45,6 @@
     "remark-math": "^6.0.0",
     "remark-parse": "^11.0.0",
     "remark-rehype": "^11.1.2",
-    "chart.js": "4.5.1",
     "svelte-spa-router": "^4.0.1",
     "unified": "^11.0.5",
     "unist-util-visit": "^5.1.0"