fix: enhance error handling in Nav component and provide fallback for Supabase client initialization

Fix: 7-day session config, plan gating on scanner (10 free/month), file upload button, upgrade modal when limit hit

v3: All missing features — auth callback, settings, PDF export, email (Resend), JWT auth, /api/history, SaulLM integration, extension icons, Supabase 0.10 breaking changes, Stripe v22

🛡️ ClauseGuard v1.0 — Full codebase: Extension + Website + API + ML