feat: add hover animation to conversation context menu button (#2168)

* feat: add hover animation to conversation context menu button

Adds visual feedback when hovering over the three-dot menu button in conversation tiles.
This makes it clear that the submenu is being targeted rather than the parent item.

Changes:
- Added rounded hover background with smooth transition
- Increased clickable area for better UX
- Supports both light and dark themes

Closes #2097

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

Co-Authored-By: Claude <noreply@anthropic.com>

* Update frontend/src/conversation/ConversationTile.tsx

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

frontend/src/conversation/ConversationTile.tsx

@@ -248,7 +248,7 @@ export default function ConversationTile({
                   event.stopPropagation();
                   setOpen(!isOpen);
                 }}
-                className="mr-2 flex w-4 justify-center"
+                className="mr-2 flex h-6 w-6 items-center justify-center rounded-full transition-colors duration-200 hover:bg-gray-200 dark:hover:bg-gray-700"
               >
                 <img src={threeDots} width={8} alt="menu" />
               </button>