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/login/+page.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+    import { m } from '$lib/paraglide/messages.js';
     let { form }: { form?: { error?: string; success?: boolean } } = $props();
 </script>
 
@@ -17,17 +18,17 @@
 
             <!-- Card -->
             <div class="bg-white border border-brand-sand rounded-sm shadow-sm p-8">
-                <h1 class="font-sans text-sm font-bold uppercase tracking-widest text-brand-navy mb-6">Anmelden</h1>
+                <h1 class="font-sans text-sm font-bold uppercase tracking-widest text-brand-navy mb-6">{m.login_heading()}</h1>
 
                 <form method="POST" action="?/login" class="space-y-5">
                     <div>
-                        <label for="username" class="block text-xs font-bold font-sans uppercase tracking-widest text-gray-500 mb-1.5">Benutzername</label>
+                        <label for="username" class="block text-xs font-bold font-sans uppercase tracking-widest text-gray-500 mb-1.5">{m.login_label_username()}</label>
                         <input type="text" name="username" id="username" required autocomplete="username"
                                class="block w-full border border-gray-300 py-2.5 px-3 text-sm font-serif text-brand-navy placeholder-gray-400 focus:border-brand-navy focus:ring-1 focus:ring-brand-navy focus:outline-none" />
                     </div>
 
                     <div>
-                        <label for="password" class="block text-xs font-bold font-sans uppercase tracking-widest text-gray-500 mb-1.5">Passwort</label>
+                        <label for="password" class="block text-xs font-bold font-sans uppercase tracking-widest text-gray-500 mb-1.5">{m.login_label_password()}</label>
                         <input type="password" name="password" id="password" required autocomplete="current-password"
                                class="block w-full border border-gray-300 py-2.5 px-3 text-sm font-serif text-brand-navy placeholder-gray-400 focus:border-brand-navy focus:ring-1 focus:ring-brand-navy focus:outline-none" />
                     </div>
@@ -38,7 +39,7 @@
 
                     <button type="submit"
                             class="w-full bg-brand-navy text-white py-2.5 text-xs font-bold uppercase tracking-widest font-sans hover:bg-brand-navy/90 transition-colors mt-2">
-                        Anmelden
+                        {m.login_btn_submit()}
                     </button>
                 </form>
             </div>