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/new/+page.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
 let { form } = $props();
 </script>
 
@@ -22,9 +23,9 @@ let { form } = $props();
 					d="M10 19l-7-7m0 0l7-7m-7 7h18"
 				/>
 			</svg>
-			Zurück zur Übersicht
+			{m.btn_back_to_overview()}
 		</a>
-		<h1 class="font-serif text-3xl text-brand-navy">Neue Person</h1>
+		<h1 class="font-serif text-3xl text-brand-navy">{m.persons_new_heading()}</h1>
 	</div>
 
 	{#if form?.error}
@@ -34,13 +35,13 @@ let { form } = $props();
 	<form method="POST">
 		<div class="rounded-sm border border-brand-sand bg-white p-6 shadow-sm">
 			<h2 class="mb-5 text-xs font-bold tracking-widest text-gray-400 uppercase">
-				Angaben zur Person
+				{m.persons_section_details()}
 			</h2>
 
 			<div class="grid grid-cols-1 gap-5 md:grid-cols-2">
 				<div>
 					<label for="firstName" class="mb-1 block text-sm font-medium text-gray-700"
-						>Vorname *</label
+						>{m.form_label_first_name()} *</label
 					>
 					<input
 						id="firstName"
@@ -53,7 +54,7 @@ let { form } = $props();
 
 				<div>
 					<label for="lastName" class="mb-1 block text-sm font-medium text-gray-700"
-						>Nachname *</label
+						>{m.form_label_last_name()} *</label
 					>
 					<input
 						id="lastName"
@@ -66,13 +67,13 @@ let { form } = $props();
 
 				<div class="md:col-span-2">
 					<label for="alias" class="mb-1 block text-sm font-medium text-gray-700"
-						>Rufname / Alias</label
+						>{m.form_label_alias()}</label
 					>
 					<input
 						id="alias"
 						name="alias"
 						type="text"
-						placeholder="z.B. Oma Frieda, Onkel Karl…"
+						placeholder={m.form_placeholder_alias()}
 						class="block w-full rounded border border-gray-300 p-2 text-sm shadow-sm focus:border-brand-navy focus:ring-brand-navy"
 					/>
 				</div>
@@ -87,13 +88,13 @@ let { form } = $props();
 				href="/persons"
 				class="text-sm font-medium text-gray-500 transition-colors hover:text-brand-navy"
 			>
-				Abbrechen
+				{m.btn_cancel()}
 			</a>
 			<button
 				type="submit"
 				class="rounded bg-brand-navy px-6 py-2 text-sm font-bold tracking-widest text-white uppercase transition-colors hover:bg-brand-navy/80"
 			>
-				Erstellen
+				{m.btn_create()}
 			</button>
 		</div>
 	</form>