feat(shared/ui): implement brutalist Button component replacing placeholder

src/shared/ui/Button/Button.stories.svelte

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+<script module>
+import { defineMeta } from '@storybook/addon-svelte-csf';
+import Button from './Button.svelte';
+
+const { Story } = defineMeta({
+    title: 'Shared/Button',
+    component: Button,
+    tags: ['autodocs'],
+    parameters: {
+        layout: 'centered',
+        docs: { description: { component: 'Brutalist CTA button. Hard shadow, uppercase, hover translates 2px.' } },
+    },
+    argTypes: {
+        variant: { control: 'select', options: ['primary', 'secondary', 'outline', 'ghost'] },
+        size:    { control: 'select', options: ['sm', 'md', 'lg'] },
+    },
+});
+</script>
+
+<Story name="Primary" args={{ variant: 'primary', size: 'md' }}>
+    {#snippet template(args)}
+        <Button {...args}>Primary</Button>
+    {/snippet}
+</Story>
+
+<Story name="Secondary" args={{ variant: 'secondary', size: 'md' }}>
+    {#snippet template(args)}
+        <Button {...args}>Secondary</Button>
+    {/snippet}
+</Story>
+
+<Story name="Outline" args={{ variant: 'outline', size: 'md' }}>
+    {#snippet template(args)}
+        <Button {...args}>Outline</Button>
+    {/snippet}
+</Story>
+
+<Story name="Ghost" args={{ variant: 'ghost', size: 'md' }}>
+    {#snippet template(args)}
+        <Button {...args}>Ghost</Button>
+    {/snippet}
+</Story>
+
+<Story name="All sizes">
+    {#snippet template()}
+        <div class="flex gap-4 items-center">
+            <Button size="sm">Small</Button>
+            <Button size="md">Medium</Button>
+            <Button size="lg">Large</Button>
+        </div>
+    {/snippet}
+</Story>
+
+<Story name="Disabled">
+    {#snippet template()}
+        <Button disabled>Disabled</Button>
+    {/snippet}
+</Story>

src/shared/ui/Button/Button.svelte

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+<!--
+    Component: Button
+    Brutalist button. 3px border, hard shadow on hover, uppercase, zero radius.
+    Hover: translates 2px down-right with reduced shadow for press feel.
+-->
+<script lang="ts">
+import { cn } from '$shared/lib/cn';
+import type { Snippet } from 'svelte';
+import type { HTMLButtonAttributes } from 'svelte/elements';
+import type { ButtonSize, ButtonVariant } from './types';
+
+interface Props extends HTMLButtonAttributes {
+    /** Visual style @default 'primary' */
+    variant?: ButtonVariant;
+    /** Size preset @default 'md' */
+    size?: ButtonSize;
+    children?: Snippet;
+    class?: string;
+}
+
+let {
+    variant = 'primary',
+    size = 'md',
+    children,
+    class: className,
+    type = 'button',
+    disabled,
+    ...rest
+}: Props = $props();
+
+const base =
+    'brutal-border transition-all duration-200 ' +
+    'hover:translate-x-[2px] hover:translate-y-[2px] hover:shadow-[6px_6px_0_var(--carbon-black)] ' +
+    'active:translate-x-[4px] active:translate-y-[4px] active:shadow-[4px_4px_0_var(--carbon-black)] ' +
+    'uppercase tracking-wider inline-flex items-center justify-center ' +
+    'disabled:opacity-50 disabled:cursor-not-allowed disabled:pointer-events-none';
+
+const variantStyles: Record<ButtonVariant, string> = {
+    primary:   'brutal-shadow bg-burnt-oxide text-ochre-clay',
+    secondary: 'brutal-shadow bg-slate-indigo text-ochre-clay',
+    outline:   'brutal-shadow bg-transparent text-carbon-black',
+    ghost:     'bg-ochre-clay text-carbon-black shadow-none hover:shadow-[6px_6px_0_var(--carbon-black)]',
+};
+
+const sizeStyles: Record<ButtonSize, string> = {
+    sm: 'px-4 py-2 text-sm',
+    md: 'px-6 py-3 text-base',
+    lg: 'px-8 py-4 text-lg',
+};
+
+const classes = $derived(cn(base, variantStyles[variant], sizeStyles[size], className));
+</script>
+
+<button {type} {disabled} class={classes} {...rest}>
+    {#if children}
+        {@render children()}
+    {/if}
+</button>

src/shared/ui/Button/Button.test.ts

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+import { render } from '@testing-library/svelte';
+import { describe, expect, it } from 'vitest';
+import Button from './Button.svelte';
+
+describe('Button', () => {
+    it('renders children', () => {
+        const { getByText } = render(Button, { props: { children: () => 'Click me' } });
+        expect(getByText('Click me')).toBeTruthy();
+    });
+
+    it('is disabled when disabled prop is true', () => {
+        const { container } = render(Button, { props: { disabled: true } });
+        expect(container.querySelector('button')?.disabled).toBe(true);
+    });
+
+    it('applies primary variant styles', () => {
+        const { container } = render(Button, { props: { variant: 'primary' } });
+        expect(container.querySelector('button')?.className).toContain('bg-burnt-oxide');
+    });
+});

src/shared/ui/Button/index.ts

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+export { default as Button } from './Button.svelte';
+export type { ButtonVariant, ButtonSize } from './types';

src/shared/ui/Button/types.ts

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+export type ButtonVariant = 'primary' | 'secondary' | 'outline' | 'ghost';
+export type ButtonSize = 'sm' | 'md' | 'lg';

src/shared/ui/index.ts

@@ -1,2 +1,2 @@
-export { default as Button } from './Button.svelte';
-export { default as Input } from './Input.svelte';
+export * from './Badge';
+export * from './Button';