feat(wallet): ensure proper session cleanup on disconnect

[Wisdom2788]

Summary

Implements secure wallet disconnection with full session cleanup per issue #38.

What was built

• store/walletStore.ts — Zustand store holding wallet state (address, isConnected, chainId) with a clearWalletState action and shared WALLET_STORAGE_KEY constant
• services/walletService.ts — API service layer that notifies the backend to invalidate the wallet session via POST /api/wallet/disconnect
• hooks/useWallet.ts — orchestration hook: calls service → clears Zustand state → removes localStorage cache → redirects to /login. Cleanup runs unconditionally via finally so a network failure can never leave stale wallet data on the client
• components/wallet/DisconnectButton.tsx — UI component consuming useWallet, renders wallet address + active disconnect button when connected, disabled state when not connected
• app/(dashboard)/admin/page.tsx — wired DisconnectButton into admin dashboard
• store/__tests__/walletStore.test.ts + hooks/__tests__/useWallet.test.ts — 10 unit tests covering all disconnect scenarios including API failure resilience

Acceptance Criteria

• ✅ Disconnecting wallet strips all public keys from DOM — Zustand clearWalletState resets address to null
• ✅ Zustand state cleared on disconnect
• ✅ localStorage wallet cache removed on disconnect
• ✅ User redirected to /login (safe page) after disconnect
• ✅ Strict Component → Hook → Service layered architecture
• ✅ Backend API called via walletService.disconnect() — no inline mock objects
• ✅ 10/10 unit tests passing (screenshot attached)
• ✅ Implementation screenshot attached

Screenshots

Closes #38

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