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/src/lib/components/ui/dialog/dialog.svelte

@@ -0,0 +1,7 @@
+<script lang="ts">
+	import { Dialog as DialogPrimitive } from "bits-ui";
+
+	let { open = $bindable(false), ...restProps }: DialogPrimitive.RootProps = $props();
+</script>
+
+<DialogPrimitive.Root bind:open {...restProps} />