feat(campaigns): inline Leads on detail + remove View Leads button

QA ask: leads should be the default view on campaign detail, not behind tab navigation, with campaign metrics (budget / funnel / source) kept visible alongside.

- drop the Overview/Leads tabs
- render LeadTable filtered to campaignLeads on the left
- Campaign Details card + Conversion Funnel + Source Breakdown
  pinned on the right as a sticky sidebar
- hero: remove 'View Leads' button (was duplicate nav now)
- LeadActivitySlideout wired for row click-through

src/components/campaigns/campaign-hero.tsx

@@ -91,13 +91,6 @@ export const CampaignHero = ({ campaign }: CampaignHeroProps) => {
                             View on Platform
                         </Button>
                     )}
-                    <Button
-                        color="primary"
-                        size="sm"
-                        href={`/leads`}
-                    >
-                        View Leads
-                    </Button>
                 </div>
             </div>
         </div>