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/button/index.ts

@@ -0,0 +1,17 @@
+import Root, {
+	type ButtonProps,
+	type ButtonSize,
+	type ButtonVariant,
+	buttonVariants,
+} from "./button.svelte";
+
+export {
+	Root,
+	type ButtonProps as Props,
+	//
+	Root as Button,
+	buttonVariants,
+	type ButtonProps,
+	type ButtonSize,
+	type ButtonVariant,
+};