fix: update sticky headers and explore border for flex layout

- Change notifications header sticky position from top: 48px to top: 0
- Change explore search container sticky position from top: 48px to top: 0
- Constrain explore page border between search and tabs to feed max-width

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[e-mail]>

src/components/pages/grain-explore.js

···
     }
     .search-container {
       position: sticky;
-      top: 48px;
+      top: 0;
       background: var(--color-bg-primary);
       padding: var(--space-sm);
       z-index: 10;
+    }
+    .search-container::after {
+      content: '';
+      display: block;
+      max-width: var(--feed-max-width);
+      margin: 0 auto;
       border-bottom: 1px solid var(--color-border);
+      margin-top: var(--space-sm);
     }
     .search-container grain-input {
       max-width: var(--feed-max-width);

src/components/pages/grain-notifications.js

···
       padding: var(--space-sm);
       border-bottom: 1px solid var(--color-border);
       position: sticky;
-      top: 48px;
+      top: 0;
       background: var(--color-bg-primary);
       z-index: 10;
     }