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/vite.config.ts

@@ -2,6 +2,7 @@ import { defineConfig } from "vite";
 import { svelte } from "@sveltejs/vite-plugin-svelte";
 import tailwindcss from "@tailwindcss/vite";
 import { compression } from "vite-plugin-compression2";
+import path from "node:path";
 
 // https://vite.dev/config/
 export default defineConfig({
@@ -21,6 +22,11 @@ export default defineConfig({
     }),
   ],
   base: "/ui/",
+  resolve: {
+    alias: {
+      $lib: path.resolve(__dirname, "./src/lib"),
+    },
+  },
   build: {
     outDir: "../internal/server/ui_dist",
     assetsDir: "assets",