feat: add mobile bottom action bar with reply and email navigation

- Move Reply, Reply All, Forward from toolbar to fixed bottom bar on mobile
- Add Next/Previous email navigation buttons to the bottom bar
- Style bottom bar to match existing NavigationRail horizontal pattern
- Add onNavigateNext/onNavigatePrev props to EmailViewer
- Wire up email list index-based navigation in page.tsx
- Add padding to email content area to prevent bottom bar overlap

locales/en/common.json

@@ -175,6 +175,8 @@
     "set_color": "Set tag",
     "tag": "Tag",
     "more_actions": "More actions",
+    "previous": "Prev",
+    "next": "Next",
     "move_to": "Move to...",
     "remove_color": "Remove tag",
     "more_count": "+{count} more",
@@ -251,7 +253,9 @@
       "delete": "Delete (# or Del)",
       "star": "Star (s)",
       "unstar": "Unstar (s)",
-      "compose": "Compose (c)"
+      "compose": "Compose (c)",
+      "previous": "Previous email",
+      "next": "Next email"
     },
     "spam": {
       "button_title": "Report spam",