feat: add wallet transaction history page

[hillbertwang]

Summary

Closes #6.

Adds a new /history page for wallet transaction history. The page uses the connected Freighter wallet address and reads recent Stellar testnet operations from Horizon instead of displaying static mock data.

• Fetches recent account operations from NEXT_PUBLIC_HORIZON_URL, defaulting to Stellar testnet Horizon
• Classifies outgoing wallet operations as contributions, incoming payments as payouts, and other wallet activity as group joins
• Displays transaction type, timestamp, amount/asset, and transaction hash
• Links each transaction hash to Stellar Expert
• Handles wallet connect, Freighter install, loading, empty, error, and refresh states
• Adds a History link to the navbar

Test plan

• git diff --cached --check passed before commit
• npm install could not complete because this standalone repository depends on @sorosave/sdk@workspace:* but the SDK package is not present in this repo
• corepack pnpm install also failed with ERR_PNPM_WORKSPACE_PKG_NOT_FOUND for the same missing workspace package

/claim #6