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/src/routes/persons/+page.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 import { goto } from '$app/navigation';
+import { m } from '$lib/paraglide/messages.js';
 
 let { data } = $props();
 
@@ -20,16 +21,16 @@ function handleSearch(e: Event) {
 		class="mb-10 flex flex-col justify-between gap-6 border-b border-brand-navy/10 pb-6 md:flex-row md:items-end"
 	>
 		<div>
-			<h1 class="font-serif text-3xl font-medium text-brand-navy">Personenverzeichnis</h1>
+			<h1 class="font-serif text-3xl font-medium text-brand-navy">{m.persons_heading()}</h1>
 			<p class="mt-2 max-w-xl font-sans text-sm text-brand-navy/60">
-				Durchsuchen Sie den Index aller erfassten Personen im Familienarchiv.
+				{m.persons_subtitle()}
 			</p>
 			<a
 				href="/persons/new"
 				class="mt-3 inline-flex items-center gap-1 text-sm font-medium text-brand-navy/60 transition-colors hover:text-brand-navy"
 			>
 				<img src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Add/Add-General-MD.svg" alt="" aria-hidden="true" class="h-4 w-4" />
-				Neue Person
+				{m.persons_btn_new()}
 			</a>
 		</div>
 
@@ -40,7 +41,7 @@ function handleSearch(e: Event) {
 				<input
 					id="search"
 					type="text"
-					placeholder="Namen suchen..."
+					placeholder={m.persons_search_placeholder()}
 					value={data.q || ''}
 					oninput={handleSearch}
 					class="block w-full rounded-sm border border-gray-300 bg-white py-2.5 pr-10 pl-4 font-sans text-sm text-brand-navy placeholder-gray-400 shadow-sm focus:border-brand-navy focus:ring-1 focus:ring-brand-navy focus:outline-none"
@@ -63,8 +64,8 @@ function handleSearch(e: Event) {
 			>
 				<img src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Account-MD.svg" alt="" aria-hidden="true" class="h-6 w-6" />
 			</div>
-			<p class="font-serif text-lg text-brand-navy">Keine Personen gefunden.</p>
-			<p class="mt-1 font-sans text-sm text-gray-500">Versuchen Sie einen anderen Suchbegriff.</p>
+			<p class="font-serif text-lg text-brand-navy">{m.persons_empty_heading()}</p>
+			<p class="mt-1 font-sans text-sm text-gray-500">{m.persons_empty_text()}</p>
 		</div>
 	{:else}
 		<div class="grid grid-cols-1 gap-6 sm:grid-cols-2 md:grid-cols-3 lg:grid-cols-4">