feat: implement i18n — extract all UI strings, add EN + ES-MX translations, add language selector

Extract all hardcoded German strings from every .svelte file and component
into Paraglide message keys. Add complete translations for all keys in
messages/en.json (English) and messages/es.json (Spanish/Mexico).

Changes:
- messages/de.json: 100+ keys covering navigation, buttons, form labels,
  placeholders, section headings, empty states, and error messages
- messages/en.json, messages/es.json: complete translations for all keys
- project.inlang/settings.json: change baseLocale from "en" to "de"
- +layout.svelte: add DE/EN/ES language selector in header using setLocale();
  active language is bold, choice persists via Paraglide cookie strategy
- All 10 route pages + 3 shared components: replace hardcoded German with m.key()
- e2e/lang.spec.ts: E2E tests for language selector visibility, switching,
  persistence across navigation, and active state highlighting

Closes #2
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/e2e/lang.spec.ts

@@ -0,0 +1,37 @@
+import { test, expect } from '@playwright/test';
+
+test.describe('Language selector', () => {
+	test('shows DE, EN, ES buttons in the header', async ({ page }) => {
+		await page.goto('/');
+		await expect(page.getByRole('banner').getByRole('button', { name: 'DE' })).toBeVisible();
+		await expect(page.getByRole('banner').getByRole('button', { name: 'EN' })).toBeVisible();
+		await expect(page.getByRole('banner').getByRole('button', { name: 'ES' })).toBeVisible();
+	});
+
+	test('switching to EN translates the navigation', async ({ page }) => {
+		await page.goto('/');
+		await page.getByRole('banner').getByRole('button', { name: 'EN' }).click();
+		await expect(page.getByRole('navigation').getByRole('link', { name: 'Documents' })).toBeVisible();
+		await expect(page.getByRole('navigation').getByRole('link', { name: 'Persons' })).toBeVisible();
+	});
+
+	test('language choice persists after navigation', async ({ page }) => {
+		await page.goto('/');
+		await page.getByRole('banner').getByRole('button', { name: 'EN' }).click();
+		await page.goto('/persons');
+		await expect(page.getByRole('navigation').getByRole('link', { name: 'Documents' })).toBeVisible();
+	});
+
+	test('switching back to DE restores German', async ({ page }) => {
+		await page.goto('/');
+		await page.getByRole('banner').getByRole('button', { name: 'EN' }).click();
+		await page.getByRole('banner').getByRole('button', { name: 'DE' }).click();
+		await expect(page.getByRole('navigation').getByRole('link', { name: 'Dokumente' })).toBeVisible();
+	});
+
+	test('active language button is visually highlighted', async ({ page }) => {
+		await page.goto('/');
+		const deBtn = page.getByRole('banner').getByRole('button', { name: 'DE' });
+		await expect(deBtn).toHaveClass(/font-bold/);
+	});
+});