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/lib/components/PersonMultiSelect.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
     import type { components } from '$lib/generated/api';
+    import { m } from '$lib/paraglide/messages.js';
     type Person = components['schemas']['Person'];
 
     interface Props {
@@ -76,7 +77,7 @@
                     type="button"
                     onclick={() => removePerson(person.id)}
                     class="text-brand-navy/50 hover:text-red-500 focus:outline-none ml-0.5"
-                    aria-label="Entfernen"
+                    aria-label={m.comp_multiselect_remove()}
                 >
                     <svg class="w-3 h-3" fill="none" stroke="currentColor" viewBox="0 0 24 24">
                         <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12"/>
@@ -92,7 +93,7 @@
             bind:value={searchTerm}
             oninput={handleInput}
             onfocus={() => { updateDropdownPosition(); showDropdown = true; }}
-            placeholder={selectedPersons.length === 0 ? 'Namen tippen...' : ''}
+            placeholder={selectedPersons.length === 0 ? m.comp_multiselect_placeholder() : ''}
             class="flex-1 min-w-[120px] border-none p-1 focus:ring-0 text-sm bg-transparent outline-none"
         />
     </div>
@@ -103,7 +104,7 @@
             class="z-50 bg-white shadow-lg max-h-60 rounded-md py-1 text-base ring-1 ring-black ring-opacity-5 overflow-auto sm:text-sm"
         >
             {#if loading}
-                <div class="p-2 text-gray-500 text-sm">Suche...</div>
+                <div class="p-2 text-gray-500 text-sm">{m.comp_multiselect_loading()}</div>
             {:else}
                 {#each results as person}
                     <div