feat: folder multi-select mode with checkboxes, shows contacts from multiple folders

frontend/src/pages/ContactsList.tsx

@@ -50,6 +50,7 @@ export function ContactsListPage() {
   const [groupState, setGroupState] = useState<GroupState>(emptyGroupState);
   const [viewsOpen, setViewsOpen] = useState(true);
   const [foldersOpen, setFoldersOpen] = useState(true);
+  const [multiSelectFolders, setMultiSelectFolders] = useState<string[]>([]);
   const [savedViews, setSavedViews] = useState<{ id: string; name: string; filterState: FilterState; sortState: SortState; groupState: GroupState; selectedFilter: ContactFilter; viewMode: ContactViewMode }[]>([]);
   const [activeViewId, setActiveViewId] = useState<string | null>(null);
   const openWindow = useWindowStore((s) => s.openWindow);
@@ -113,7 +114,10 @@ export function ContactsListPage() {
   // Filter by tag client-side if tag filter is active, then apply FilterPanel conditions
   const filteredContacts = useMemo(() => {
     let result = contacts;
-    if (tagFilter) {
+    // Multi-select folder filter — show contacts from multiple folders
+    if (multiSelectFolders.length > 0) {
+      result = result.filter((c) => c.folder_id != null && multiSelectFolders.includes(c.folder_id));
+    } else if (tagFilter) {
       result = result.filter((c) => {
         if (!c.tags) return false;
         return c.tags.split(',').map((t) => t.trim()).includes(tagFilter);
@@ -124,7 +128,7 @@ export function ContactsListPage() {
     // Apply SortPanel sorting
     result = applySorting(result, sortState);
     return result;
-  }, [contacts, tagFilter, filterState, sortState]);
+  }, [contacts, tagFilter, filterState, sortState, multiSelectFolders]);
 
   // Apply GroupPanel grouping
   const groupedContacts = useMemo(() => applyGrouping(filteredContacts, groupState), [filteredContacts, groupState]);
@@ -370,7 +374,7 @@ export function ContactsListPage() {
     ];
     registerItems('contacts', items);
     return () => unregisterPlugin('contacts');
-  }, [handleSearch, handleCreate, handleSelectFilter, handleSaveView, applySavedView, deleteSavedView, t, registerItems, unregisterPlugin, selectedFilter, viewMode, sortOptions, viewModeOptions, filterState, contactType, sortState, groupState, savedViews, activeViewId]);
+  }, [handleSearch, handleCreate, handleSelectFilter, handleSaveView, applySavedView, deleteSavedView, t, registerItems, unregisterPlugin, selectedFilter, viewMode, sortOptions, viewModeOptions, filterState, contactType, sortState, groupState, savedViews, activeViewId, multiSelectFolders]);
 
   return (
     <div className="flex flex-col h-full" data-testid="contacts-list-page">
@@ -486,6 +490,8 @@ export function ContactsListPage() {
                     onSelect={handleSelectFilter}
                     tags={allTags}
                     contacts={contacts}
+                    multiSelectFolders={multiSelectFolders}
+                    onMultiSelectChange={setMultiSelectFolders}
                   />
                 </div>
               )}