feat(KB-171): constrain column widths in desktop grid and mobile flex

- Change desktop .column min-width from 260px to 0, letting the grid control sizing
- Add overflow-x: hidden to .column-body to prevent content blowout
- Use fixed width: 280px for mobile columns instead of min-width
- Add column-fixed-width tests verifying CSS constraints for desktop and mobile

packages/dashboard/app/styles.css

@@ -200,7 +200,7 @@ body {
   border-radius: var(--radius-lg);
   border: 1px solid var(--border);
   overflow: hidden;
-  min-width: 260px;
+  min-width: 0;
   min-height: 0;
   transition: border-color 0.2s;
   scroll-snap-align: center;
@@ -277,6 +277,7 @@ body {
 .column-body {
   flex: 1;
   overflow-y: auto;
+  overflow-x: hidden;
   padding: 4px 8px 8px;
   display: flex;
   flex-direction: column;
@@ -1529,7 +1530,7 @@ body {
   }
 
   .board > .column {
-    min-width: 280px;
+    width: 280px;
     flex-shrink: 0;
     scroll-snap-align: center;
   }