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/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>