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/tsconfig.json

@@ -14,7 +14,12 @@
     "noUnusedLocals": true,
     "noUnusedParameters": true,
     "noFallthroughCasesInSwitch": true,
-    "verbatimModuleSyntax": true
+    "verbatimModuleSyntax": true,
+    "baseUrl": ".",
+    "paths": {
+      "$lib": ["./src/lib"],
+      "$lib/*": ["./src/lib/*"]
+    }
   },
   "include": ["src/**/*.ts", "src/**/*.svelte"]
 }