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/components/leads/lead-table.tsx

@@ -25,6 +25,7 @@ type LeadTableProps = {
     onSort: (field: string) => void;
     onViewActivity?: (lead: Lead) => void;
     visibleColumns?: Set<string>;
+    selectionMode?: 'multiple' | 'none';
 };
 
 type TableRow = {
@@ -55,6 +56,7 @@ export const LeadTable = ({
     onSort,
     onViewActivity,
     visibleColumns,
+    selectionMode,
 }: LeadTableProps) => {
     const [expandedDupId, setExpandedDupId] = useState<string | null>(null);
 
@@ -118,7 +120,7 @@ export const LeadTable = ({
         <div className="flex flex-1 flex-col min-h-0 overflow-hidden rounded-xl ring-1 ring-secondary">
             <Table
                 aria-label="Leads table"
-                selectionMode="multiple"
+                selectionMode={selectionMode ?? 'multiple'}
                 selectionBehavior="toggle"
                 selectedKeys={selectedKeys}
                 onSelectionChange={handleSelectionChange}