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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UmuGqwNBJNEAMaWsdCDqUC

ui-svelte/components.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "https://shadcn-svelte.com/schema.json",
+  "tailwind": {
+    "css": "src/index.css",
+    "baseColor": "zinc"
+  },
+  "aliases": {
+    "components": "$lib/components",
+    "utils": "$lib/utils",
+    "ui": "$lib/components/ui",
+    "hooks": "$lib/hooks",
+    "lib": "$lib"
+  },
+  "typescript": true,
+  "registry": "https://shadcn-svelte.com/registry",
+  "iconLibrary": "lucide"
+}