feat: add refund UI, spend simulator, SLA nudges, and error boundaries

[Osuochasam]

Overview
This release introduces core user-facing features for escrow refunds and spend simulation, alongside proactive SLA notifications and resilient UI error handling to prevent page-wide crashes.

Key Changes:

Refund Request UI: Built a complete refund surface for eligible escrows. Includes a RefundCTA form (with eligibility gating, reason code selection, and a double-submission guard) and a RefundTimeline component to track the status progression of requests. Data structures and read-only fetching logic are isolated in refund.ts and useRefundEligibility.ts.

Spend Simulator Panel: Added a read-only simulation tool to the delegation detail page. Users can test hypothetical spends, and the SpendSimulatorPanel returns whether the transaction would be allowed or denied. If denied, it displays the specific bounding constraint and provides a direct, actionable remediation link (e.g., to raise a cap or renew a limit).

Proactive SLA Nudges: Implemented a notification system to warn users before pending approvals breach their SLA threshold. The useSLANudge hook polls pending requests and issues both in-app and OS notifications. It features robust deduplication, a snooze function backed by localStorage, and focus-suppression to avoid nagging active users.

Granular Error Boundaries: Shipped a reusable class component with a Tailwind-styled fallback card (supporting dark mode). Instead of an entire page crashing due to one failing widget, the boundary catches the error locally, preserves the state of interactive sibling components, provides a localized "retry" button, and exports a hook point for future Sentry ingestion.

Closes #569
Closes #572
Closes #575
Closes #586

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