feat(ui): add BottomSheet.svelte shared wrapper component

Shared wrapper for C4, C6, and future sheet flows. Handles dim overlay,
drag handle, focus trap, Escape dismiss, and backdrop click dismiss.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/BottomSheet.test.ts

@@ -0,0 +1,52 @@
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen, fireEvent } from '@testing-library/svelte';
+import userEvent from '@testing-library/user-event';
+import BottomSheet from './BottomSheet.svelte';
+
+describe('BottomSheet', () => {
+	it('is not mounted in DOM when open is false', () => {
+		render(BottomSheet, { props: { open: false, onclose: vi.fn() } });
+		expect(screen.queryByTestId('bottom-sheet')).toBeNull();
+	});
+
+	it('is mounted in DOM when open is true', () => {
+		render(BottomSheet, { props: { open: true, onclose: vi.fn() } });
+		expect(screen.getByTestId('bottom-sheet')).toBeTruthy();
+	});
+
+	it('calls onclose when close button is clicked', async () => {
+		const onclose = vi.fn();
+		render(BottomSheet, { props: { open: true, onclose } });
+		const closeBtn = screen.getByRole('button', { name: /schließen/i });
+		await userEvent.click(closeBtn);
+		expect(onclose).toHaveBeenCalledOnce();
+	});
+
+	it('calls onclose when backdrop is clicked', async () => {
+		const onclose = vi.fn();
+		render(BottomSheet, { props: { open: true, onclose } });
+		const backdrop = screen.getByTestId('sheet-backdrop');
+		await userEvent.click(backdrop);
+		expect(onclose).toHaveBeenCalledOnce();
+	});
+
+	it('calls onclose when Escape is pressed', async () => {
+		const onclose = vi.fn();
+		render(BottomSheet, { props: { open: true, onclose } });
+		await userEvent.keyboard('{Escape}');
+		expect(onclose).toHaveBeenCalledOnce();
+	});
+
+	it('drag handle has aria-hidden', () => {
+		render(BottomSheet, { props: { open: true, onclose: vi.fn() } });
+		const handle = screen.getByTestId('drag-handle');
+		expect(handle.getAttribute('aria-hidden')).toBe('true');
+	});
+
+	it('does not call onclose when Escape is pressed while closed', async () => {
+		const onclose = vi.fn();
+		render(BottomSheet, { props: { open: false, onclose } });
+		await userEvent.keyboard('{Escape}');
+		expect(onclose).not.toHaveBeenCalled();
+	});
+});