feat(branch-panel): improve context menu positioning and styling

This update enhances the context menu for branches and tags by
ensuring it fits within the viewport, preventing overflow and
improving usability. Additionally, the styling of the branch
context menu has been adjusted for better visibility and
interaction.

- Added dynamic positioning for context menus to avoid overflow
- Updated CSS for branch context menu to improve layout and usability
- Simplified context menu opening logic for better performance

src/app.css

@@ -2212,7 +2212,11 @@
     box-shadow: 0 18px 50px rgba(0,0,0,0.5);
   }
 
-  .branch-context-menu,
+  .branch-context-menu {
+    position: fixed;
+    max-height: calc(100vh - 16px);
+    overflow-y: auto;
+  }
   .history-context-menu { position: absolute; }
   .explorer-context-menu,
   .repo-tab-context-menu { position: fixed; }