chore: purge shadcn from codebase. Replace with bits-ui components and other tools

src/shared/ui/Button/Button.svelte

@@ -3,7 +3,7 @@
     design-system button. Uppercase, zero border-radius, Space Grotesk.
 -->
 <script lang="ts">
-import { cn } from '$shared/shadcn/utils/shadcn-utils';
+import clsx from 'clsx';
 import type { Snippet } from 'svelte';
 import type { HTMLButtonAttributes } from 'svelte/elements';
 import type {
@@ -71,7 +71,7 @@ let {
 const isIconOnly = $derived(!!icon && !children);
 
 const variantStyles: Record<ButtonVariant, string> = {
-    primary: cn(
+    primary: clsx(
         'bg-swiss-red text-white',
         'hover:bg-swiss-red/90',
         'active:bg-swiss-red/80',
@@ -87,7 +87,7 @@ const variantStyles: Record<ButtonVariant, string> = {
         'disabled:cursor-not-allowed',
         'disabled:transform-none',
     ),
-    secondary: cn(
+    secondary: clsx(
         'bg-surface dark:bg-paper',
         'text-swiss-black dark:text-neutral-200',
         'border border-black/10 dark:border-white/10',
@@ -98,7 +98,7 @@ const variantStyles: Record<ButtonVariant, string> = {
         'disabled:text-neutral-400 dark:disabled:text-neutral-600',
         'disabled:cursor-not-allowed',
     ),
-    outline: cn(
+    outline: clsx(
         'bg-transparent',
         'text-swiss-black dark:text-neutral-200',
         'border border-black/20 dark:border-white/20',
@@ -109,7 +109,7 @@ const variantStyles: Record<ButtonVariant, string> = {
         'disabled:text-neutral-400 dark:disabled:text-neutral-600',
         'disabled:cursor-not-allowed',
     ),
-    ghost: cn(
+    ghost: clsx(
         'bg-transparent',
         'text-secondary',
         'border border-transparent',
@@ -119,7 +119,7 @@ const variantStyles: Record<ButtonVariant, string> = {
         'disabled:text-neutral-400 dark:disabled:text-neutral-600',
         'disabled:cursor-not-allowed',
     ),
-    icon: cn(
+    icon: clsx(
         'bg-surface dark:bg-dark-bg',
         'text-secondary',
         'border border-transparent',
@@ -130,8 +130,8 @@ const variantStyles: Record<ButtonVariant, string> = {
         'disabled:text-neutral-400 dark:disabled:text-neutral-600',
         'disabled:cursor-not-allowed',
     ),
-    tertiary: cn(
-        // Font override — must come after base in cn() to win via tailwind-merge
+    tertiary: clsx(
+        // Font override — must come after base in clsx() to win via tailwind-merge
         'font-secondary font-medium normal-case tracking-normal',
         // Inactive state
         'bg-transparent',
@@ -175,7 +175,7 @@ const activeStyles: Partial<Record<ButtonVariant, string>> = {
     icon: 'bg-paper dark:bg-paper text-brand border-subtle',
 };
 
-const classes = $derived(cn(
+const classes = $derived(clsx(
     // Base
     'inline-flex items-center justify-center',
     'font-primary font-bold tracking-tight uppercase',

src/shared/ui/Button/ButtonGroup.svelte

@@ -4,7 +4,7 @@
     Use for segmented controls, view toggles, or any mutually exclusive button set.
 -->
 <script lang="ts">
-import { cn } from '$shared/shadcn/utils/shadcn-utils';
+import clsx from 'clsx';
 import type { Snippet } from 'svelte';
 import type { HTMLAttributes } from 'svelte/elements';
 
@@ -23,7 +23,7 @@ let { children, class: className, ...rest }: Props = $props();
 </script>
 
 <div
-    class={cn(
+    class={clsx(
         'flex items-center gap-1 p-1',
         'bg-surface dark:bg-dark-bg',
         'border border-subtle',