[Frontend] Optimistic UI updates for milestone actions

[Gbangbolaoluwagbemiga]

Problem

Clicking "Approve Milestone" shows a spinner and waits ~5s for Stellar confirmation. The UI feels unresponsive.

Solution

Optimistic updates: show expected state immediately, reconcile on confirmation, roll back on error.

What needs to be done

1. `ApprovalsPage` — on approve click, immediately move card to Approved state; on error roll back to Submitted
2. `DashboardPage` — start work immediately shows In Progress
3. Use TanStack Query's `onMutate` / `onError` / `onSettled` (already installed)

Acceptance criteria

• Approve action shows optimistic state instantly
• Error case rolls back with visible user feedback
• No duplicate network calls
• Works correctly with concurrent viewers