Improve mobile layout spacing and stacking

- Stack PageHeader title and actions vertically on mobile
- Make action button full-width on mobile
- Reduce padding throughout for tighter layout
- Compact empty state with less whitespace

Author: JSv4

src/stories/CorpusDetailPage.stories.tsx

@@ -402,7 +402,55 @@ const pageStyles = `
     }
 
     .corpus-main__header {
-      padding-top: 8px;
+      padding-top: 12px;
+      padding-bottom: 12px;
+    }
+
+    .corpus-main__search {
+      padding-top: 12px;
+      padding-bottom: 12px;
+    }
+
+    .corpus-main__content {
+      padding-top: 16px;
+      padding-bottom: 16px;
+    }
+
+    /* Stack header row vertically on mobile */
+    .corpus-header-row {
+      flex-direction: column;
+      align-items: stretch;
+      gap: 8px;
+    }
+
+    .corpus-mobile-toggle {
+      width: 40px;
+      margin-top: 0;
+    }
+
+    /* Stack PageHeader title and actions */
+    .corpus-header-row .oc-page-header__row {
+      flex-direction: column;
+      align-items: flex-start;
+      gap: 12px;
+    }
+
+    .corpus-header-row .oc-page-header__actions {
+      width: 100%;
+    }
+
+    .corpus-header-row .oc-page-header__actions .oc-btn {
+      width: 100%;
+      justify-content: center;
+    }
+
+    /* Compact empty state */
+    .corpus-main__inner .oc-card {
+      padding: 0;
+    }
+
+    .corpus-main__inner .oc-empty-state {
+      padding: 32px 16px;
     }
   }
 
@@ -412,6 +460,10 @@ const pageStyles = `
       min-width: 100%;
       max-width: 320px;
     }
+
+    .corpus-main__content {
+      padding: 12px;
+    }
   }
 `;