Fix lingering selection border on Activity page

The :hover rule painted the same yellow accent border as the selected
state, so deselecting a card while the cursor was still over it left the
border visible until another card was selected. Remove the hover
mechanism (also unnecessary on touch devices) so the border is driven
solely by the checked state.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

index.php

@@ -163,10 +163,6 @@ foreach ($rows as $row) {
     transition: border-color .15s, background .15s, transform .1s;
     user-select: none;
   }
-  .activity-card label:hover {
-    border-color: var(--accent);
-    background: #1e2333;
-  }
   .check-icon {
     width: 18px; height: 18px;
     border: 1.5px solid var(--border);