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/src/lib/components/ui/select/select.svelte

@@ -0,0 +1,11 @@
+<script lang="ts">
+	import { Select as SelectPrimitive } from "bits-ui";
+
+	let {
+		open = $bindable(false),
+		value = $bindable(),
+		...restProps
+	}: SelectPrimitive.RootProps = $props();
+</script>
+
+<SelectPrimitive.Root bind:open bind:value={value as never} {...restProps} />