feat(i18n): fix remaining hardcoded strings and add login page switcher

- Add 9 missing translation keys to de/en/es.json:
  doc_file_error_preview, doc_download_title, doc_tag_filter_title,
  doc_conversation_title, doc_preview_iframe_title, doc_image_alt,
  doc_no_date, person_merge_will_be_deleted, admin_user_delete_confirm

- documents/[id]/+page.svelte: replace 6 hardcoded strings with m.*()
- persons/[id]/+page.svelte: replace "wird gelöscht." and "Kein Datum"
- admin/+page.svelte: replace confirm() string with m.admin_user_delete_confirm()
- login/+page.svelte: add top-right DE/EN/ES language switcher (Option B)
  and wire existing login_* keys to the form labels

Closes #12

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -155,6 +155,17 @@
 	"admin_group_delete_confirm": "Gruppe wirklich löschen?",
 	"admin_section_new_group": "Neue Gruppe anlegen",
 	"admin_group_name_placeholder": "Gruppenname (z.B. Editoren)",
+	"admin_user_delete_confirm": "Benutzer {username} wirklich löschen?",
+
+	"doc_file_error_preview": "Vorschau konnte nicht geladen werden.",
+	"doc_download_title": "Herunterladen",
+	"doc_tag_filter_title": "Nach {name} filtern",
+	"doc_conversation_title": "Konversation anzeigen",
+	"doc_preview_iframe_title": "Dokumentvorschau",
+	"doc_image_alt": "Original-Scan",
+	"doc_no_date": "Kein Datum",
+
+	"person_merge_will_be_deleted": "wird gelöscht.",
 
 	"comp_typeahead_placeholder": "Namen tippen...",
 	"comp_typeahead_loading": "Suche...",

frontend/messages/en.json

@@ -155,6 +155,17 @@
 	"admin_group_delete_confirm": "Really delete group?",
 	"admin_section_new_group": "Create new group",
 	"admin_group_name_placeholder": "Group name (e.g. Editors)",
+	"admin_user_delete_confirm": "Really delete user {username}?",
+
+	"doc_file_error_preview": "Could not load preview.",
+	"doc_download_title": "Download",
+	"doc_tag_filter_title": "Filter by {name}",
+	"doc_conversation_title": "Show conversation",
+	"doc_preview_iframe_title": "Document Preview",
+	"doc_image_alt": "Original scan",
+	"doc_no_date": "No date",
+
+	"person_merge_will_be_deleted": "will be deleted.",
 
 	"comp_typeahead_placeholder": "Type a name...",
 	"comp_typeahead_loading": "Searching...",

frontend/messages/es.json

@@ -155,6 +155,17 @@
 	"admin_group_delete_confirm": "¿Realmente eliminar el grupo?",
 	"admin_section_new_group": "Crear nuevo grupo",
 	"admin_group_name_placeholder": "Nombre del grupo (p.ej. Editores)",
+	"admin_user_delete_confirm": "¿Realmente eliminar al usuario {username}?",
+
+	"doc_file_error_preview": "No se pudo cargar la vista previa.",
+	"doc_download_title": "Descargar",
+	"doc_tag_filter_title": "Filtrar por {name}",
+	"doc_conversation_title": "Ver conversación",
+	"doc_preview_iframe_title": "Vista previa del documento",
+	"doc_image_alt": "Escaneado original",
+	"doc_no_date": "Sin fecha",
+
+	"person_merge_will_be_deleted": "será eliminado.",
 
 	"comp_typeahead_placeholder": "Escriba un nombre...",
 	"comp_typeahead_loading": "Buscando...",

frontend/src/routes/admin/+page.svelte

@@ -202,7 +202,7 @@
 											method="POST"
 											action="?/deleteUser"
 											use:enhance={({ cancel }) => {
-												if (!confirm(`Benutzer '${user.username}' wirklich löschen?`)) {
+												if (!confirm(m.admin_user_delete_confirm({ username: user.username }))) {
 													cancel();
 												}
 												return async ({ update }) => {

frontend/src/routes/documents/[id]/+page.svelte

@@ -33,7 +33,7 @@
 
         } catch (e) {
             console.error(e);
-            error = 'Vorschau konnte nicht geladen werden.';
+            error = m.doc_file_error_preview();
         } finally {
             isLoading = false;
         }
@@ -87,7 +87,7 @@
 					href={fileUrl}
 					download={doc.originalFilename}
 					class="text-brand-navy bg-brand-sand/50 hover:bg-brand-mint border border-transparent p-2 rounded transition"
-					title="Download"
+					title={m.doc_download_title()}
 				>
 					<img src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Download-MD.svg" alt="" aria-hidden="true" class="w-5 h-5" />
 				</a>
@@ -163,7 +163,7 @@
 											<a
 												href="/?tag={encodeURIComponent(tag.name)}"
 												class="inline-flex items-center px-2 py-0.5 rounded text-xs font-bold uppercase tracking-wide bg-brand-sand/50 text-brand-navy hover:bg-brand-navy hover:text-white transition-colors"
-												title="Nach '{tag.name}' filtern"
+												title={m.doc_tag_filter_title({ name: tag.name })}
 											>
 												{tag.name}
 											</a>
@@ -241,7 +241,7 @@
 											<a
 												href="/conversations?senderId={doc.sender.id}&receiverId={receiver.id}"
 												class="text-gray-300 hover:text-brand-mint transition"
-												title="Konversation anzeigen"
+												title={m.doc_conversation_title()}
 											>
 												<img src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Chat-MD.svg" alt="" aria-hidden="true" class="w-5 h-5" />
 											</a>
@@ -341,14 +341,14 @@
 			{:else if fileUrl && doc.originalFilename.toLowerCase().endsWith('.pdf')}
 				<iframe
 					src={fileUrl}
-					title="Document Preview"
+					title={m.doc_preview_iframe_title()}
 					class="w-full h-full border-none bg-white"
 				></iframe>
 			{:else if fileUrl}
 				<div class="w-full h-full flex items-center justify-center overflow-auto p-8">
 					<img
 						src={fileUrl}
-						alt="Original Scan"
+						alt={m.doc_image_alt()}
 						class="max-w-full max-h-full object-contain shadow-2xl"
 					/>
 				</div>

frontend/src/routes/login/+page.svelte

@@ -1,12 +1,34 @@
 <script lang="ts">
     import { m } from '$lib/paraglide/messages.js';
+    import { setLocale, getLocale } from '$lib/paraglide/runtime';
+
     let { form }: { form?: { error?: string; success?: boolean } } = $props();
+
+    const locales = ['DE', 'EN', 'ES'] as const;
+    const localeMap = { DE: 'de', EN: 'en', ES: 'es' } as const;
+    const activeLocale = $derived(getLocale().toUpperCase());
 </script>
 
-<div class="min-h-screen bg-white flex flex-col">
+<div class="relative min-h-screen bg-white flex flex-col">
     <!-- DGB purple accent strip -->
     <div class="h-1 bg-brand-purple"></div>
 
+    <!-- Language switcher -->
+    <div class="absolute top-4 right-4 flex items-center gap-1">
+        {#each locales as locale (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>
+
     <div class="flex-1 flex items-center justify-center px-4">
         <div class="w-full max-w-sm">
             <!-- Logo -->

frontend/src/routes/persons/[id]/+page.svelte

@@ -189,7 +189,7 @@
 
                 {#if showMergeConfirm}
                     <p class="mt-3 text-sm text-red-700 bg-red-50 border border-red-200 rounded px-3 py-2">
-                        {m.person_merge_warning()} <strong>{person.firstName} {person.lastName}</strong> wird gelöscht.
+                        {m.person_merge_warning()} <strong>{person.firstName} {person.lastName}</strong> {m.person_merge_will_be_deleted()}
                     </p>
                 {/if}
             </form>
@@ -225,7 +225,7 @@
                                             {doc.title || doc.originalFilename}
                                         </div>
                                         <div class="flex items-center text-xs font-sans text-gray-500 mt-0.5 space-x-2">
-                                            <span>{doc.documentDate ? new Intl.DateTimeFormat('de-DE', { day: 'numeric', month: 'long', year: 'numeric' }).format(new Date(doc.documentDate + 'T12:00:00')) : 'Kein Datum'}</span>
+                                            <span>{doc.documentDate ? new Intl.DateTimeFormat('de-DE', { day: 'numeric', month: 'long', year: 'numeric' }).format(new Date(doc.documentDate + 'T12:00:00')) : m.doc_no_date()}</span>
                                             {#if doc.location}
                                                 <span class="text-brand-mint">•</span>
                                                 <span>{doc.location}</span>