Add retry stats, form validation, column resize, and user activity timeline

[miss-yusrah]

closes #487
closes #498
closes #501
closes #506

Summary

• Notification retry statistics: Record and expose retry counts via calculation helpers, API endpoints (/api/schedule/execution-metrics, /retry-distribution, /retry-statistics), and a dashboard Retry Stats panel with distribution breakdown.
• Form validation feedback: Inline validation on the subscription form (and preference contact fields) with immediate messages, field highlighting, and accessible aria-invalid / aria-describedby / role="alert" wiring.
• Table column resizing: Event Explorer columns are resizable with stable grid layout; widths persist in localStorage.
• User activity timeline: New chronological timeline for subscription/notification management actions, including empty-state handling and dashboard tab.

What changed

Task 1 — Notification Retry Statistics

• Added listener/src/services/retry-statistics.ts with pure calc helpers and payload builder.
• Wired metrics endpoints in events-server.ts and NotificationAPI.getRetryStatistics().
• Added dashboard RetryStatisticsPanel + API client + unit/UI tests.

Task 2 — Form Validation Feedback

• SubscriptionForm validates on blur/submit; invalid fields are highlighted immediately.
• Preference email/Telegram fields now use accessible inline errors.
• Expanded frontend validation tests.

Task 3 — Table Column Resizing

• EventExplorerTable supports drag resize handles, CSS grid stability, and persisted widths.
• Added helper/UI tests for load/persist/clamp behavior.

Task 4 — User Activity Timeline

• Added UserActivityTimeline + userActivityApi with chronological sorting and empty state.
• Exposed under the User Activity dashboard tab.

Test plan

• listener: npm test -- --testPathPattern=retry-statistics
• dashboard: npm test -- --testPathPattern='RetryStatisticsPanel|EventExplorerTable|UserActivityTimeline|retryStatistics'
• frontend: npx vitest run src/components/__tests__/SubscriptionForm.test.tsx
• Manually open dashboard tabs: Retry Stats, User Activity, resize Event Explorer columns and reload to confirm persistence
• Confirm subscription form shows inline errors on blur for short group name / invalid usages

Notes

• Branch: feature/ux-stats
• Do not commit frontend/node_modules/ if it appears untracked
• No commit/push performed (per request)

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