feat(pitch): showcase mode — per-investor toggle hides financial/investor slides for customer demos

Adds is_showcase boolean to pitch_investors; when set, filters out financials,
the ask, cap table, assumptions, finanzplan, risks, and intro-presenter slides.
Slide navigation is fully dynamic — progress bar and counts update accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pitch-deck/lib/hooks/useAuth.ts

@@ -11,6 +11,7 @@ export interface Investor {
   last_login_at: string | null
   login_count: number
   created_at: string
+  is_showcase: boolean
 }
 
 export function useAuth() {