feat: tree grouping for list and cards views (matching table view)

frontend/src/components/contacts/ContactList.tsx

@@ -475,10 +475,48 @@ export function ContactList({
       </li>
     );
 
+    // Group header renderer for list view
+    const renderListGroupHeader = (group: GroupedContacts) => {
+      const isExpanded = expandedGroups.has(group.key);
+      return (
+        <div
+          key={`group-header-${group.key}`}
+          className="flex items-center gap-2 px-3 py-2 bg-secondary-50 border-b border-secondary-200"
+        >
+          <button
+            onClick={() => toggleGroup(group.key)}
+            className="flex items-center justify-center w-5 h-5 hover:bg-secondary-200 rounded transition-colors flex-shrink-0"
+            aria-label={isExpanded ? 'Collapse group' : 'Expand group'}
+          >
+            {isExpanded ? (
+              <ChevronDown className="w-3.5 h-3.5" />
+            ) : (
+              <ChevronRight className="w-3.5 h-3.5" />
+            )}
+          </button>
+          <span className="font-medium text-sm text-secondary-700 truncate">{group.label}</span>
+          <span className="text-secondary-400 text-xs flex-shrink-0">({group.contacts.length})</span>
+        </div>
+      );
+    };
+
     return (
       <div className="flex flex-col h-full" data-testid="contact-list-view">
         <div ref={scrollRef} className="flex-1 overflow-y-auto">
-          {shouldVirtualize ? (
+          {isGrouped && groupedContacts ? (
+            <div>
+              {groupedContacts.map((group) => (
+                <div key={group.key}>
+                  {renderListGroupHeader(group)}
+                  {expandedGroups.has(group.key) && (
+                    <ul className="divide-y divide-secondary-100" role="list">
+                      {group.contacts.map((contact) => renderContactItem(contact))}
+                    </ul>
+                  )}
+                </div>
+              ))}
+            </div>
+          ) : shouldVirtualize ? (
             <ul className="divide-y divide-secondary-100" role="list" style={{ height: rowVirtualizer.getTotalSize(), position: 'relative' }}>
               {rowVirtualizer.getVirtualItems().map((virtualRow) => {
                 const contact = contacts[virtualRow.index];
@@ -826,10 +864,48 @@ export function ContactList({
     </div>
   );
 
+  // Group header renderer for cards view
+  const renderCardsGroupHeader = (group: GroupedContacts) => {
+    const isExpanded = expandedGroups.has(group.key);
+    return (
+      <div
+        key={`group-header-${group.key}`}
+        className="flex items-center gap-2 px-3 py-2 bg-secondary-50 border-b border-secondary-200 mb-3 sticky top-0 z-10"
+      >
+        <button
+          onClick={() => toggleGroup(group.key)}
+          className="flex items-center justify-center w-5 h-5 hover:bg-secondary-200 rounded transition-colors flex-shrink-0"
+          aria-label={isExpanded ? 'Collapse group' : 'Expand group'}
+        >
+          {isExpanded ? (
+            <ChevronDown className="w-3.5 h-3.5" />
+          ) : (
+            <ChevronRight className="w-3.5 h-3.5" />
+          )}
+        </button>
+        <span className="font-medium text-sm text-secondary-700 truncate">{group.label}</span>
+        <span className="text-secondary-400 text-xs flex-shrink-0">({group.contacts.length})</span>
+      </div>
+    );
+  };
+
   return (
     <div className="flex flex-col h-full" data-testid="contact-cards-view">
       <div ref={scrollRef} className="flex-1 overflow-y-auto p-3">
-        {shouldVirtualize ? (
+        {isGrouped && groupedContacts ? (
+          <div className="space-y-4">
+            {groupedContacts.map((group) => (
+              <div key={group.key}>
+                {renderCardsGroupHeader(group)}
+                {expandedGroups.has(group.key) && (
+                  <div className="grid grid-cols-1 sm:grid-cols-2 gap-3">
+                    {group.contacts.map((contact) => renderContactCard(contact))}
+                  </div>
+                )}
+              </div>
+            ))}
+          </div>
+        ) : shouldVirtualize ? (
           <div style={{ height: rowVirtualizer.getTotalSize(), position: 'relative' }}>
             <div className="grid grid-cols-1 sm:grid-cols-2 gap-3" style={{ position: 'absolute', top: 0, left: 0, right: 0 }}>
               {rowVirtualizer.getVirtualItems().map((virtualRow) => {