Show fallback banner when current-locale translation is missing

DocumentViewer computes viewLocale and isFallback at mount; banner
links authenticated owners to the editor for the current UI locale.
Adds documents.fallback_notice + locale_names to all 16 lang files
(en+ja human-translated, others mirror en for now).

src/resources/views/livewire/document-viewer.blade.php

@@ -1,4 +1,20 @@
 <div class="max-w-4xl mx-auto p-4 sm:p-6 lg:p-8">
+    @if($isFallback)
+        <div class="mb-4 p-4 bg-amber-50 border border-amber-300 rounded-md flex flex-col sm:flex-row sm:items-center sm:justify-between gap-3">
+            <p class="text-sm text-amber-800">
+                {{ __('messages.documents.fallback_notice', ['locale' => __('messages.locale_names.' . $viewLocale)]) }}
+            </p>
+            @auth
+                @can('update', $document)
+                    <a href="{{ route('documents.translations.edit', ['document' => $document, 'locale' => app()->getLocale()]) }}"
+                       class="inline-flex items-center justify-center px-3 py-2 bg-amber-600 text-white text-sm font-medium rounded-md hover:bg-amber-700">
+                        {{ __('messages.documents.add_translation') }}
+                    </a>
+                @endcan
+            @endauth
+        </div>
+    @endif
+
     <!-- Document Header -->
     <div class="mb-6 sm:mb-8">
         <div class="flex flex-col sm:flex-row sm:items-start sm:justify-between gap-4 mb-4">