feat: implement tablet 2-column grid layout for issue and PR lists

Add grid layout at >= 640px breakpoint for #issue-list and #pull-list
containers, matching the existing .card-grid tablet behavior. Cards
render in a 2-column grid on tablet while maintaining single-column
on mobile.

Closes leeworks-agents/gitea-mobile#105

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/style.css

@@ -510,13 +510,17 @@ a:active {
     max-width: 960px;
   }
 
-  .card-grid {
+  .card-grid,
+  #issue-list,
+  #pull-list {
     display: grid;
     grid-template-columns: repeat(2, 1fr);
     gap: var(--spacing-sm);
   }
 
-  .card-grid .card {
+  .card-grid .card,
+  #issue-list .card,
+  #pull-list .card {
     margin-bottom: 0;
   }
 }