feat(person): add conversations quick-link (#20)

Add a "Konversationen anzeigen" link to the person detail page header
that navigates to /conversations?senderId={id}, pre-filling the person
as Person A. Includes i18n in de/en/es and an E2E test.

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

frontend/e2e/persons.spec.ts

@@ -72,6 +72,19 @@ test.describe('New person', () => {
 	});
 });
 
+test.describe('Person detail — conversations link', () => {
+	test('has a conversations link that pre-fills the person', async ({ page }) => {
+		await page.goto('/persons');
+		const firstLink = page.locator('a[href^="/persons/"]').first();
+		const href = await firstLink.getAttribute('href');
+		const personId = href!.split('/persons/')[1];
+		await firstLink.click();
+		const convLink = page.getByRole('link', { name: /Konversationen/i });
+		await expect(convLink).toBeVisible();
+		await expect(convLink).toHaveAttribute('href', `/conversations?senderId=${personId}`);
+	});
+});
+
 test.describe('Conversations', () => {
 	test('shows the empty state when no persons are selected', async ({ page }) => {
 		await page.goto('/conversations');