Add Stripe payment method for tier upgrades and paid onboarding

Description

Add a payment method integration using Stripe. This is needed for two scenarios:

1. Tier Upgrades: When existing users upgrade to higher tiers, they need to add a payment method
2. Paid Onboarding: When new users onboard directly to a paid tier, they need to set up payment during the onboarding flow

Requirements

• Integrate Stripe for payment processing
• Add payment method UI for tier upgrades
• Add payment method UI during paid tier onboarding
• Handle payment method storage and management