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-description.svelte

@@ -0,0 +1,17 @@
+<script lang="ts">
+	import { Dialog as DialogPrimitive } from "bits-ui";
+	import { cn } from "$lib/utils.js";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		...restProps
+	}: DialogPrimitive.DescriptionProps = $props();
+</script>
+
+<DialogPrimitive.Description
+	bind:ref
+	data-slot="dialog-description"
+	class={cn("text-muted-foreground *:[a]:hover:text-foreground text-sm *:[a]:underline *:[a]:underline-offset-3", className)}
+	{...restProps}
+/>