feat: implement vesting escrow management UI

[Majormaxx]

Summary

Implements the Vesting Escrow Management UI as specified in Issue #145. Builds a full management page for the vesting_escrow Soroban contract, supporting both admin grant creation and beneficiary claim workflows.

Problem

The vesting_escrow contract was deployed but had no frontend interface. Admins could not create grants and beneficiaries could not claim vested tokens without using the CLI or raw XDR tooling.

Changes

File	Action	Description
frontend/src/services/vestingService.ts	NEW	Soroban contract integration service wrapping get_config, get_claimable_amount, initialize, claim, and clawback calls. Builds and simulates XDR envelopes via RPC.
frontend/src/pages/VestingManagement.tsx	NEW	Full management page with two views: Dashboard (active grant details, vested progress bar, claim button) and Admin Setup (form to initialize a new grant with all parameters). Errors surfaced via sonner toasts and the shared TransactionSimulationPanel.
frontend/src/App.tsx	MODIFIED	Added /vesting route with ErrorBoundary wrapper.
frontend/src/components/AppNav.tsx	MODIFIED	Added Vesting navigation link with Lock icon between Cross-Asset and History.

Acceptance Criteria

• Page lists active vesting grant fetched from the contract
• Progress bar reflects vested vs. unvested token amounts
• "Claim" button invokes the contract claim entry point and shows simulation result
• Admin form to create a new grant (start date, cliff, duration, amount) submits via signed XDR
• Errors surfaced via the toast notification system

Testing

• npx tsc --noEmit passes with zero errors
• All existing tests unaffected (no breaking changes to public APIs)
• Route accessible at /vesting and nav link renders correctly

Closes #145.