feat: add landing page with hero, features, stats, and CTA sections

[olawaleakanbi035-maker]

closes #387

Implements the main landing page for Nevo with a full hero section, features, stats, and calls
to action.

Changes

• Hero section with gradient background, headline, subheading, and Create Pool / Browse Pools
  CTAs
• Stats bar with mock trust indicators (pools created, total donated, contributors, avg fee)
• Features grid (4 cards: Open Pools, Transparent Tracking, Trustless Withdrawals, Low Fees)
• "How it works" 3-step section
• Bottom CTA section
• Fully responsive layout (mobile → tablet → desktop)
• Semantic HTML with ARIA labels throughout

Also fixed

• Added missing zustand dependency
• Added stubs for app/stellar-wallets-kit.ts and lib/stellar.ts (referenced by existing store
  files, causing build failure)

Testing

• next build passes with no errors or type errors

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