fix: Leads page cleanup — remove tabs, checkboxes, inline header

- Remove New/My Leads/All Leads tabs — redundant now that contacts
  are on a separate page; all leads shown as a flat list
- Remove row checkboxes (selectionMode="none") — bulk actions weren't
  wired to any backend and confused QA
- Move Search + Columns + Export into the header row alongside the
  title — cleaner single-row layout
- Remove BulkActionBar + AssignModal + WhatsAppSendModal + MarkSpamModal
  imports and JSX — dead code without checkboxes
- LeadTable: new selectionMode prop (default "multiple" for back-compat)
- Same cleanup on Contacts page (no checkboxes)

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

src/pages/contacts.tsx

@@ -35,7 +35,6 @@ export const ContactsPage = () => {
     const { leads, leadActivities } = useData();
     const [searchQuery, setSearchQuery] = useState('');
     const [currentPage, setCurrentPage] = useState(1);
-    const [selectedIds, setSelectedIds] = useState<string[]>([]);
     const [sortField, setSortField] = useState('createdAt');
     const [sortDirection, setSortDirection] = useState<'asc' | 'desc'>('desc');
     const [activityLead, setActivityLead] = useState<Lead | null>(null);
@@ -142,8 +141,9 @@ export const ContactsPage = () => {
                 <div className="flex-1 overflow-y-auto px-4 pt-3">
                     <LeadTable
                         leads={paged}
-                        selectedIds={selectedIds}
-                        onSelectionChange={setSelectedIds}
+                        selectedIds={[]}
+                        onSelectionChange={() => {}}
+                        selectionMode="none"
                         sortField={sortField}
                         sortDirection={sortDirection}
                         onSort={handleSort}
@@ -157,7 +157,7 @@ export const ContactsPage = () => {
                         <PaginationPageDefault
                             page={currentPage}
                             total={totalPages}
-                            onPageChange={(page) => { setCurrentPage(page); setSelectedIds([]); }}
+                            onPageChange={(page) => { setCurrentPage(page); }}
                         />
                     </div>
                 )}