feat: make filters dropdown multi-column layout

Converts the single-column stacked filters popover into a 2-column grid
layout. Status filters appear in the left column, Priority + Assignee in
the right column. On mobile (<640px), falls back to single column.
Popover width increased from 320px to 480px with viewport-aware max to
prevent overflow.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/components/IssuesList.tsx

@@ -225,7 +225,7 @@ export function IssuesList({
                 )}
               </Button>
             </PopoverTrigger>
-            <PopoverContent align="end" className="w-80 p-0">
+            <PopoverContent align="end" className="w-[min(480px,calc(100vw-2rem))] p-0">
               <div className="p-3 space-y-3">
                 <div className="flex items-center justify-between">
                   <span className="text-sm font-medium">Filters</span>
@@ -264,6 +264,8 @@ export function IssuesList({
 
                 <div className="border-t border-border" />
 
+                {/* Multi-column filter sections */}
+                <div className="grid grid-cols-1 sm:grid-cols-2 gap-x-4 gap-y-3">
                   {/* Status */}
                   <div className="space-y-1">
                     <span className="text-xs text-muted-foreground">Status</span>
@@ -281,8 +283,8 @@ export function IssuesList({
                     </div>
                   </div>
 
-                <div className="border-t border-border" />
-
+                  {/* Priority + Assignee stacked in right column */}
+                  <div className="space-y-3">
                     {/* Priority */}
                     <div className="space-y-1">
                       <span className="text-xs text-muted-foreground">Priority</span>
@@ -302,8 +304,6 @@ export function IssuesList({
 
                     {/* Assignee */}
                     {agents && agents.length > 0 && (
-                  <>
-                    <div className="border-t border-border" />
                       <div className="space-y-1">
                         <span className="text-xs text-muted-foreground">Assignee</span>
                         <div className="space-y-0.5 max-h-32 overflow-y-auto">
@@ -318,9 +318,10 @@ export function IssuesList({
                           ))}
                         </div>
                       </div>
-                  </>
                     )}
                   </div>
+                </div>
+              </div>
             </PopoverContent>
           </Popover>