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/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 }) => {