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/+layout.svelte

@@ -3,9 +3,15 @@
     import { enhance } from '$app/forms';
     import { page } from '$app/state';
     import { onMount } from 'svelte';
+    import { m } from '$lib/paraglide/messages.js';
+    import { setLocale, getLocale } from '$lib/paraglide/runtime';
 
     let { children } = $props();
 
+    const locales = ['DE', 'EN', 'ES'] as const;
+    const localeMap = { DE: 'de', EN: 'en', ES: 'es' } as const;
+    const activeLocale = $derived(getLocale().toUpperCase());
+
     const isAdmin = $derived(page.data.user?.groups.some((g: { permissions: string[] }) => g.permissions.includes('ADMIN')));
 
     // Set after client-side hydration completes. Used by E2E tests to know the
@@ -40,7 +46,7 @@
                                     ? 'text-brand-navy bg-brand-purple/15 rounded'
                                     : 'text-gray-500 hover:text-brand-navy hover:bg-brand-sand/60 rounded'}"
                             >
-                                Dokumente
+                                {m.nav_documents()}
                             </a>
 
                             <a
@@ -50,7 +56,7 @@
                                     ? 'text-brand-navy bg-brand-purple/15 rounded'
                                     : 'text-gray-500 hover:text-brand-navy hover:bg-brand-sand/60 rounded'}"
                             >
-                                Personen
+                                {m.nav_persons()}
                             </a>
 
                             <a
@@ -60,7 +66,7 @@
                                     ? 'text-brand-navy bg-brand-purple/15 rounded'
                                     : 'text-gray-500 hover:text-brand-navy hover:bg-brand-sand/60 rounded'}"
                             >
-                                Konversationen
+                                {m.nav_conversations()}
                             </a>
                             {#if isAdmin}
                                 <a
@@ -70,25 +76,41 @@
                                         ? 'text-brand-navy bg-brand-purple/15 rounded'
                                         : 'text-gray-500 hover:text-brand-navy hover:bg-brand-sand/60 rounded'}"
                                 >
-                                    Admin
+                                    {m.nav_admin()}
                                 </a>
                             {/if}
                         </nav>
                     </div>
 
                     <!-- Right Side -->
-                    <div class="flex items-center">
+                    <div class="flex items-center gap-3">
+                        <!-- Language selector -->
+                        <div class="flex items-center gap-1 border-r border-gray-200 pr-3">
+                            {#each locales as locale}
+                                <button
+                                    type="button"
+                                    onclick={() => setLocale(localeMap[locale])}
+                                    class="text-xs font-sans tracking-widest px-1.5 py-1 transition-colors
+                                        {activeLocale === locale
+                                            ? 'font-bold text-brand-navy'
+                                            : 'font-normal text-gray-400 hover:text-brand-navy'}"
+                                >
+                                    {locale}
+                                </button>
+                            {/each}
+                        </div>
                         <form action="/logout" method="POST" use:enhance>
                             <button
                                 type="submit"
                                 class="inline-flex items-center gap-1.5 text-xs text-gray-400 hover:text-brand-navy font-bold uppercase font-sans tracking-widest px-3 py-2 transition-colors"
                             >
                                 <img src="/degruyter-icons/Simple/Small-16px/SVG/Action/Account-SM.svg" alt="" aria-hidden="true" class="w-4 h-4 opacity-50" />
-                                Abmelden
+                                {m.nav_logout()}
                             </button>
                         </form>
                     </div>
                 </div>
+
             </div>
         </header>
     {/if}