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...",