refactor: unified maint modal with pre-step support, OTP-gated campaign clear

- Extended MaintAction with needsPreStep + clientSideHandler
- MaintOtpModal supports pre-step content before OTP (campaign selection)
- Removed standalone ClearCampaignLeadsModal — all maint actions go through one modal
- 4-step import wizard with Untitled UI Select for mapping
- DynamicTable className passthrough

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

src/components/campaigns/lead-import-wizard.tsx

@@ -314,8 +314,8 @@ export const LeadImportWizard = ({ isOpen, onOpenChange }: LeadImportWizardProps
                                             {noPhoneCount > 0 && <span className="text-error-primary">{noPhoneCount} no phone</span>}
                                         </div>
 
-                                        {/* Table — fills remaining space, body scrolls */}
-                                        <div className="flex-1 min-h-0 overflow-auto px-4 pt-2">
+                                        {/* Table — fills remaining space, header pinned, body scrolls */}
+                                        <div className="flex flex-1 flex-col min-h-0 px-4 pt-2">
                                             <DynamicTable<DynamicRow>
                                                 columns={[
                                                     ...mapping.filter(m => m.leadField).map(m => ({