[Suggestion] Add budget guardrails and alerts #39

Is your feature request related to a problem? Please describe.
Quota bars and projections are informative, but users cannot define spending limits or receive proactive warnings.

Describe the solution you'd like
Support monthly, daily, and session budgets with remaining allowance, projected overage, configurable warning thresholds, and opt-in toast alerts.

Additional context
Execution order: 7, after #38. Integrate with the configuration work in #12; begin with soft warnings rather than automatic blocking.