budgetTracker: Add onOverspend callback when spending exceeds budget ceiling

[Kingsman-99]

Description

budgetTracker tracks cumulative spending against a budget but silently exceeds limits. Contributors should add an onOverspend(overspentBy: number, budgetId: string) callback option that fires whenever a new payment pushes the total above the configured budget ceiling, so callers can trigger warnings or block further payments.

Acceptance Criteria

• An onOverspend callback option is accepted by the tracker's configuration
• The callback fires exactly once per payment that crosses the budget ceiling (not once per subsequent payment)
• overspentBy is the positive difference between the new total and the budget limit
• Existing tests continue to pass

Context

• Target file: src/lib/budgetTracker.ts

[guptakumarranjeet150]

Hello @maintainers,

I can tackle budgetTracker: Add onOverspend callback when spending exceeds budget ceiling. Proposed implementation roadmap:

• Analysis: Inspect current behavior and edge cases in Stellar-split/split-app
• Implementation: Deliver a clean, modular solution for budgetTracker: Add onOverspend callback when spending exceeds budget ceiling strictly following Stellar-split/split-app conventions.
• Testing: Cover changes with automated tests ensuring CI passes smoothly.
• PR: Atomic commit with clean git hygiene and passing CI

Looking forward to contributing—please feel free to assign!