fix: modal scrollbar clips rounded corners + sidebar hover jump (v56)

- modal-card: overflow:hidden + flex column; modal-body scrolls instead,
  so native scrollbar never overlaps the 20px border-radius corners
- cal-item-remove / cal-item-bell: display:none→flex replaced with
  opacity:0/pointer-events:none→1/auto, eliminating reflow on hover

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/js/version.js

@@ -1,2 +1,2 @@
 // Increment APP_VERSION with every code change
-export const APP_VERSION = 'v55';
+export const APP_VERSION = 'v56';