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/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>