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/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 -->