refactor(document): extract DocumentMobileMenu from DocumentTopBar

Second step of the Phase 5 split. The kebab dropdown — including
clickOutside handling and its own mobileMenuOpen state — becomes its
own component named after its visual region. The mobile snippet
duplication inside DocumentTopBar is removed; the component owns its
mobile-specific markup.

TDD: DocumentMobileMenu.svelte.test.ts (7 tests) was red first. The
component then made it green (kebab trigger, dropdown open/close on
click, transcribe button gated on canWrite × isPdf × !transcribeMode,
download link gated on filePath). DocumentTopBar wraps the new
component in a md:hidden div so responsive behaviour is unchanged.
Existing 18-test DocumentTopBar suite still passes.

Refs #496.

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

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

@@ -0,0 +1,91 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import DocumentMobileMenu from './DocumentMobileMenu.svelte';
+
+afterEach(cleanup);
+
+const baseProps = {
+	canWrite: false,
+	isPdf: false,
+	transcribeMode: false,
+	filePath: null as string | null,
+	originalFilename: 'brief.pdf' as string | null,
+	fileUrl: ''
+};
+
+describe('DocumentMobileMenu', () => {
+	it('renders the kebab trigger button with the more-actions aria-label', async () => {
+		render(DocumentMobileMenu, { props: { ...baseProps, filePath: 'docs/x.pdf' } });
+
+		await expect.element(page.getByRole('button', { name: /weitere aktionen/i })).toBeVisible();
+	});
+
+	it('starts with the dropdown closed (aria-expanded=false)', async () => {
+		render(DocumentMobileMenu, { props: { ...baseProps, filePath: 'docs/x.pdf' } });
+
+		await expect
+			.element(page.getByRole('button', { name: /weitere aktionen/i }))
+			.toHaveAttribute('aria-expanded', 'false');
+	});
+
+	it('opens the dropdown when the trigger is clicked', async () => {
+		render(DocumentMobileMenu, { props: { ...baseProps, filePath: 'docs/x.pdf' } });
+
+		await page.getByRole('button', { name: /weitere aktionen/i }).click();
+
+		await expect
+			.element(page.getByRole('button', { name: /weitere aktionen/i }))
+			.toHaveAttribute('aria-expanded', 'true');
+	});
+
+	it('shows the transcribe action inside the open menu when canWrite, isPdf, and not in transcribe mode', async () => {
+		render(DocumentMobileMenu, {
+			props: { ...baseProps, canWrite: true, isPdf: true, filePath: 'docs/x.pdf' }
+		});
+
+		await page.getByRole('button', { name: /weitere aktionen/i }).click();
+
+		await expect.element(page.getByRole('button', { name: /transkribieren/i })).toBeVisible();
+	});
+
+	it('hides the transcribe action when already in transcribeMode', async () => {
+		render(DocumentMobileMenu, {
+			props: {
+				...baseProps,
+				canWrite: true,
+				isPdf: true,
+				transcribeMode: true,
+				filePath: 'docs/x.pdf'
+			}
+		});
+
+		await page.getByRole('button', { name: /weitere aktionen/i }).click();
+
+		await expect
+			.element(page.getByRole('button', { name: /transkribieren/i }))
+			.not.toBeInTheDocument();
+	});
+
+	it('shows the download link inside the open menu when filePath is present', async () => {
+		render(DocumentMobileMenu, {
+			props: { ...baseProps, filePath: 'docs/x.pdf', fileUrl: '/api/docs/x' }
+		});
+
+		await page.getByRole('button', { name: /weitere aktionen/i }).click();
+
+		await expect.element(page.getByRole('link', { name: /herunterladen/i })).toBeVisible();
+	});
+
+	it('omits the download link when filePath is null', async () => {
+		render(DocumentMobileMenu, {
+			props: { ...baseProps, canWrite: true, isPdf: true }
+		});
+
+		await page.getByRole('button', { name: /weitere aktionen/i }).click();
+
+		await expect
+			.element(page.getByRole('link', { name: /herunterladen/i }))
+			.not.toBeInTheDocument();
+	});
+});