feat(web): add scroll to last message button

Add intelligent floating scroll button for long conversations that:
- Only appears when scrolling down (direction-aware)
- Auto-hides after 3 seconds of inactivity
- Stays visible on hover to prevent accidental disappearance
- Uses consistent design patterns with repo styling
- Includes proper accessibility features

🤖 Generated with [opencode](https://opencode.ai)

Co-Authored-By: Jeremy Osih <osih.jeremy@gmail.com>
Co-Authored-By: opencode <noreply@opencode.ai>

packages/web/src/components/share.module.css

@@ -760,3 +760,38 @@
     }
   }
 }
+
+.scrollButton {
+  position: fixed;
+  bottom: 2rem;
+  right: 2rem;
+  width: 2.5rem;
+  height: 2.5rem;
+  border-radius: 0.25rem;
+  border: 1px solid var(--sl-color-divider);
+  background-color: var(--sl-color-bg-surface);
+  color: var(--sl-color-text-secondary);
+  cursor: pointer;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.1);
+  transition: all 0.15s ease, opacity 0.5s ease;
+  z-index: 100;
+  appearance: none;
+  opacity: 1;
+
+  &:hover {
+    color: var(--sl-color-text);
+    border-color: var(--sl-color-hairline);
+    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
+  }
+
+  &:active {
+    transform: translateY(1px);
+  }
+
+  svg {
+    display: block;
+  }
+}