feat(FN-817): add expand button and swipe-friendly touch behavior for board cards

- Add expand button to TaskCard for opening task detail modal on mobile
- Implement CSS scroll-snap on board columns for swipe-friendly navigation
- Add touch-action and overscroll-behavior styles for smoother mobile scrolling
- Add comprehensive tests for mobile scroll snap behavior and expand button
- Document expand button and swipe-friendly board behavior in README

packages/dashboard/app/styles.css

@@ -809,7 +809,7 @@ body {
     transform var(--transition-fast),
     opacity var(--transition-normal);
   user-select: none;
-  touch-action: pan-y;
+  touch-action: pan-x pan-y;
 }
 .card:hover {
   background: var(--card-hover);
@@ -5090,6 +5090,22 @@ body {
     width: 16px;
     height: 16px;
   }
+
+  /* Card expand button: always visible on mobile (no hover) */
+  .card-expand-btn {
+    opacity: 1;
+    width: 44px;
+    height: 44px;
+    margin-right: -8px;
+    margin-top: -8px;
+    margin-bottom: -8px;
+    border-radius: var(--radius-md);
+  }
+
+  .card-expand-btn svg {
+    width: 16px;
+    height: 16px;
+  }
 }
 
 /* === Tablet Responsive Tier (769px–1024px) === */