fix(sidebar): min-height approach for action buttons, no overlap or jump

Instead of absolute positioning (which caused icons to overlay text),
use min-height:40px on .cal-item so the row is always tall enough for
28px mini-btns. Buttons stay in the flex flow with display:none when
hidden — no space reserved, no text truncation, no height shift 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 = 'v57';
+export const APP_VERSION = 'v58';