feat(employee-portal): add vesting escrow claim section

[Majormaxx]

Summary

• Adds VestingClaimSection component to EmployeePortal.tsx that fetches the employee's live claimable balance from the vesting_escrow Soroban contract via rpc.Server.simulateTransaction() on get_claimable_amount()
• Claim button is disabled when the claimable amount is zero or the wallet is disconnected; enabled with the employee's accrued balance shown in real time
• On click, the claim transaction is simulated and signed via useSorobanContract (which internally uses useWalletSigning) before on-chain submission and confirmation polling
• On confirmed success, triggers notifySuccess and immediately refreshes the claimable balance
• Claim history section lists completed claims sourced from the backend event indexer via claimService.getEmployeeClaims

Test plan

• Connect a wallet — the "Vested Token Claim" card should appear and show claimable balance
• With zero claimable amount, the Claim Tokens button is disabled
• With a non-zero claimable amount, the button is enabled and triggers the wallet signing flow
• After a successful claim, the claimable amount updates to 0 and a success notification appears
• If no VITE_VESTING_ESCROW_CONTRACT_ID is set, the section renders nothing (safe no-op)
• Claim history rows appear for any status: 'claimed' records from the backend

Closes #156

[drips-wave]

@Majormaxx Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits