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/conversations/+page.svelte

@@ -2,6 +2,7 @@
     import { goto } from '$app/navigation';
     import PersonTypeahead from '$lib/components/PersonTypeahead.svelte';
     import { untrack } from 'svelte';
+    import { m } from '$lib/paraglide/messages.js';
 
     let { data } = $props();
 
@@ -39,9 +40,9 @@
 <div class="max-w-5xl mx-auto py-10 px-4">
 	<!-- Page Header -->
 	<div class="mb-8 border-b border-brand-navy/10 pb-4">
-		<h1 class="text-3xl font-serif font-medium text-brand-navy">Konversationen</h1>
+		<h1 class="text-3xl font-serif font-medium text-brand-navy">{m.conv_heading()}</h1>
 		<p class="text-brand-navy/60 font-sans text-sm mt-2">
-			Verfolgen Sie den Schriftverkehr zwischen zwei Personen chronologisch.
+			{m.conv_subtitle()}
 		</p>
 	</div>
 
@@ -54,7 +55,7 @@
 			>
 				<PersonTypeahead
 					name="senderId"
-					label="Person A (Absender)"
+					label={m.conv_label_person_a()}
 					bind:value={senderId}
 					initialName={data.initialValues.senderName}
 					onchange={() => applyFilters()}
@@ -67,7 +68,7 @@
 			>
 				<PersonTypeahead
 					name="receiverId"
-					label="Person B (Empfänger)"
+					label={m.conv_label_person_b()}
 					bind:value={receiverId}
 					initialName={data.initialValues.receiverName}
 					onchange={() => applyFilters()}
@@ -81,7 +82,7 @@
 				<label
 					for="dateFrom"
 					class="block text-xs font-bold uppercase tracking-widest text-gray-500 mb-2"
-					>Zeitraum von</label
+					>{m.conv_label_from()}</label
 				>
 				<input
 					id="dateFrom"
@@ -97,7 +98,7 @@
 				<label
 					for="dateTo"
 					class="block text-xs font-bold uppercase tracking-widest text-gray-500 mb-2"
-					>Zeitraum bis</label
+					>{m.conv_label_to()}</label
 				>
 				<input
 					id="dateTo"
@@ -114,8 +115,8 @@
 					onclick={toggleSort}
 					class="w-full flex items-center justify-center h-[42px] border border-brand-sand text-xs font-bold uppercase tracking-wide text-brand-navy hover:bg-brand-navy hover:text-white transition-colors"
 				>
-					<span class="mr-2">Sortierung:</span>
-					<span>{sortDir === 'DESC' ? 'Neueste zuerst' : 'Älteste zuerst'}</span>
+					<span class="mr-2">{m.conv_sort_label()}</span>
+					<span>{sortDir === 'DESC' ? m.conv_sort_newest() : m.conv_sort_oldest()}</span>
 					<svg
 						class="w-4 h-4 ml-2 transform {sortDir === 'ASC'
 							? 'rotate-180'
@@ -147,15 +148,15 @@
 					/></svg
 				>
 			</div>
-			<p class="text-brand-navy font-serif text-lg">Wählen Sie zwei Personen aus</p>
-			<p class="text-gray-500 font-sans text-sm mt-1">Die Korrespondenz wird hier angezeigt.</p>
+			<p class="text-brand-navy font-serif text-lg">{m.conv_empty_heading()}</p>
+			<p class="text-gray-500 font-sans text-sm mt-1">{m.conv_empty_text()}</p>
 		</div>
 	{:else if data.documents.length === 0}
 		<div
 			class="flex flex-col items-center justify-center py-24 bg-white border border-brand-sand rounded-sm text-center shadow-sm"
 		>
-			<p class="text-brand-navy font-serif">Keine Dokumente gefunden.</p>
-			<p class="text-gray-400 text-sm mt-2">Versuchen Sie, den Zeitraum anzupassen.</p>
+			<p class="text-brand-navy font-serif">{m.conv_no_results_heading()}</p>
+			<p class="text-gray-400 text-sm mt-2">{m.conv_no_results_text()}</p>
 		</div>
 	{:else}
 		<!-- CHAT CONTAINER -->