feat: integrate scroll-to-top button in timeline

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

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

src/components/pages/grain-timeline.js

···
 import '../organisms/grain-comment-sheet.js';
 import '../molecules/grain-pull-to-refresh.js';
 import '../atoms/grain-spinner.js';
+import '../atoms/grain-scroll-to-top.js';
 
 export class GrainTimeline extends LitElement {
   static properties = {
···
     this._showScrollTop = window.scrollY > 150;
   }
 
+  async #handleScrollTop() {
+    if (this._refreshing) return;
+
+    window.scrollTo({ top: 0, behavior: 'smooth' });
+
+    // Wait for scroll to complete before refreshing
+    await new Promise(resolve => setTimeout(resolve, 400));
+
+    await this.#handleRefresh();
+  }
+
   render() {
     return html`
       <grain-feed-layout>
···
           @close=${this.#handleCommentSheetClose}
         ></grain-comment-sheet>
       </grain-feed-layout>
+      <grain-scroll-to-top
+        ?visible=${this._showScrollTop}
+        @scroll-top=${this.#handleScrollTop}
+      ></grain-scroll-to-top>
     `;
   }
 }