test(document): expand FileSwitcherStrip coverage

Adds prev/next button click safety, ArrowRight + ArrowLeft + ArrowDown
keyboard navigation through chips with wrap-around.

5 new tests covering ~10 branches.

Refs #496.

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

frontend/src/lib/document/FileSwitcherStrip.svelte.test.ts

@@ -119,4 +119,91 @@ describe('FileSwitcherStrip', () => {
 		const announcer = document.querySelector('[aria-live="polite"]');
 		expect(announcer?.textContent).toContain('Ein Brief.pdf');
 	});
+
+	it('clicking prev button does not throw with no scroll target available', async () => {
+		render(FileSwitcherStrip, {
+			props: {
+				files: [makeEntry('f1', 'A.pdf')],
+				activeId: 'f1',
+				onSelect: () => {},
+				onRemove: () => {}
+			}
+		});
+
+		const prevBtn = Array.from(document.querySelectorAll('button')).find((b) =>
+			/vorherige|prev/i.test(b.getAttribute('aria-label') ?? '')
+		) as HTMLButtonElement;
+		expect(() => prevBtn.click()).not.toThrow();
+	});
+
+	it('clicking next button does not throw', async () => {
+		render(FileSwitcherStrip, {
+			props: {
+				files: [makeEntry('f1', 'A.pdf')],
+				activeId: 'f1',
+				onSelect: () => {},
+				onRemove: () => {}
+			}
+		});
+
+		const nextBtn = Array.from(document.querySelectorAll('button')).find((b) =>
+			/nächste|next/i.test(b.getAttribute('aria-label') ?? '')
+		) as HTMLButtonElement;
+		expect(() => nextBtn.click()).not.toThrow();
+	});
+
+	it('navigates with ArrowRight key on focused chip', async () => {
+		render(FileSwitcherStrip, {
+			props: {
+				files: [makeEntry('f1', 'A'), makeEntry('f2', 'B'), makeEntry('f3', 'C')],
+				activeId: 'f1',
+				onSelect: () => {},
+				onRemove: () => {}
+			}
+		});
+
+		const f1 = document.querySelector('[data-chip-id="f1"]') as HTMLElement;
+		f1.focus();
+		f1.dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowRight', bubbles: true }));
+
+		await new Promise((r) => setTimeout(r, 30));
+		expect(document.activeElement?.getAttribute('data-chip-id')).toBe('f2');
+	});
+
+	it('navigates with ArrowLeft key on focused chip (wraps around)', async () => {
+		render(FileSwitcherStrip, {
+			props: {
+				files: [makeEntry('f1', 'A'), makeEntry('f2', 'B')],
+				activeId: 'f1',
+				onSelect: () => {},
+				onRemove: () => {}
+			}
+		});
+
+		const f1 = document.querySelector('[data-chip-id="f1"]') as HTMLElement;
+		f1.focus();
+		f1.dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowLeft', bubbles: true }));
+
+		await new Promise((r) => setTimeout(r, 30));
+		// ArrowLeft from index 0 wraps to last (f2)
+		expect(document.activeElement?.getAttribute('data-chip-id')).toBe('f2');
+	});
+
+	it('ArrowDown is treated as ArrowRight (vertical key alias)', async () => {
+		render(FileSwitcherStrip, {
+			props: {
+				files: [makeEntry('f1', 'A'), makeEntry('f2', 'B')],
+				activeId: 'f1',
+				onSelect: () => {},
+				onRemove: () => {}
+			}
+		});
+
+		const f1 = document.querySelector('[data-chip-id="f1"]') as HTMLElement;
+		f1.focus();
+		f1.dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowDown', bubbles: true }));
+
+		await new Promise((r) => setTimeout(r, 30));
+		expect(document.activeElement?.getAttribute('data-chip-id')).toBe('f2');
+	});
 });