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/lang/hi/messages.php

@@ -39,6 +39,13 @@
         'content_label' => 'सामग्री',
         'content_placeholder' => 'यहां अपना मार्कडाउन लिखें...',
         'saving' => 'सहेजा जा रहा है...',
+        'fallback_notice' => 'A translation in your selected language is not available. Showing the :locale version.',
+        'add_translation' => 'Add translation',
+        'translation_added' => 'Translation added.',
+        'translation_deleted' => 'Translation deleted.',
+        'set_as_default' => 'Set as default',
+        'delete_translation_blocked' => 'The default-language translation cannot be deleted.',
+        'translation_tabs_label' => 'Languages',
     ],
 
     // Quick Switcher
@@ -124,6 +131,25 @@
         'back_to_home' => 'होम पर वापस जाएं',
     ],
 
+    'locale_names' => [
+        'en' => 'English',
+        'ja' => 'Japanese',
+        'zh-CN' => 'Simplified Chinese',
+        'zh-TW' => 'Traditional Chinese',
+        'ko' => 'Korean',
+        'hi' => 'Hindi',
+        'vi' => 'Vietnamese',
+        'tr' => 'Turkish',
+        'de' => 'German',
+        'fr' => 'French',
+        'es' => 'Spanish',
+        'pt-BR' => 'Portuguese (Brazil)',
+        'ru' => 'Russian',
+        'uk' => 'Ukrainian',
+        'it' => 'Italian',
+        'pl' => 'Polish',
+    ],
+
     // Profile
     'profile' => [
         'title' => 'प्रोफ़ाइल',