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/sidebar/sidebar-menu-action.svelte

@@ -0,0 +1,37 @@
+<script lang="ts">
+	import { cn, type WithElementRef } from "$lib/utils.js";
+	import type { Snippet } from "svelte";
+	import type { HTMLButtonAttributes } from "svelte/elements";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		showOnHover = false,
+		children,
+		child,
+		...restProps
+	}: WithElementRef<HTMLButtonAttributes> & {
+		child?: Snippet<[{ props: Record<string, unknown> }]>;
+		showOnHover?: boolean;
+	} = $props();
+
+	const mergedProps = $derived({
+		class: cn(
+			"text-sidebar-foreground ring-sidebar-ring hover:bg-sidebar-accent hover:text-sidebar-accent-foreground peer-hover/menu-button:text-sidebar-accent-foreground absolute top-1.5 right-1 aspect-square w-5 rounded-md p-0 peer-data-[size=default]/menu-button:top-1.5 peer-data-[size=lg]/menu-button:top-2.5 peer-data-[size=sm]/menu-button:top-1 focus-visible:ring-2 [&>svg]:size-4 flex items-center justify-center outline-hidden transition-transform group-data-[collapsible=icon]:hidden after:absolute after:-inset-2 md:after:hidden [&>svg]:shrink-0",
+			showOnHover &&
+				"peer-data-active/menu-button:text-sidebar-accent-foreground group-focus-within/menu-item:opacity-100 group-hover/menu-item:opacity-100 data-open:opacity-100 md:opacity-0",
+			className
+		),
+		"data-slot": "sidebar-menu-action",
+		"data-sidebar": "menu-action",
+		...restProps,
+	});
+</script>
+
+{#if child}
+	{@render child({ props: mergedProps })}
+{:else}
+	<button bind:this={ref} {...mergedProps}>
+		{@render children?.()}
+	</button>
+{/if}