[UI/UX] Cap active toast queue to 3 with error coalescing

[nanaf6203-bit]

Problem Statement

src/components/ui/toast (presumed; uses sonner) shows overlapping toasts during error storms. When the structured logger flushes a batch with 30 errors, 30 toasts pile up off-screen.

Acceptance Criteria

• Toast queue max depth = 3
• Errors coalesce into a single "5 errors since your last view" toast
• User can dismiss the queue with one click without affecting underlying logs

Implementation Notes

sonner has a toastOptions and a toast.promise pattern. Configure a custom id strategy so duplicates merge.

Files Likely Affected

• src/components/toast/
• src/utils/structuredLogger.ts

Difficulty / Effort

• Difficulty: Easy
• Effort: T-Shirt S

Labels

ux, priority:medium