[codex] implement notification system

[DorianZheng]

Summary

Adds a configurable Dune notification system with main-process delivery, renderer settings, throttling, quiet hours, and notification history.

What Changed

• added a new Electron main-process notifications module with shared types, macOS delivery, Telegram delivery, rolling history, throttling, and idle-agent polling
• wired workflow, agent-error, and budget-warning events into the notification manager from the main/runtime layer
• added notification settings IPC in main.ts, preload exposure, desktop bridge types, and a new Notifications section in Settings
• built a notifications settings panel with trigger/channel toggles, Telegram chat id entry, Do Not Disturb controls, auto-save, and history clearing
• added focused unit tests for notification manager/history plus renderer/preload/settings coverage for the new surface

Why

Users currently have to watch the Dune UI continuously to notice review/acceptance transitions, agent failures, budget warnings, or long idle periods. This change adds configurable alerts across desktop and Telegram channels while avoiding noisy repeats.

Impact

• users can opt into the supported notification triggers and channels from Settings
• alerts are suppressed during configured quiet hours and throttled to at most one per item per 5 minutes
• the settings panel shows the last 50 delivered notifications in-session

Validation

• pnpm typecheck
• pnpm vitest run src/electron/preload.test.ts src/renderer/features/settings/config/settings-sections.test.ts src/electron/main/notifications/notification-history.test.ts src/electron/main/notifications/notification-manager.test.ts src/renderer/features/settings/notifications/NotificationsSettingsPanel.test.tsx src/electron/main/runtime/desktop-runtime-controller.test.ts

Notes

• pnpm vitest run src/electron/main/runtime/agent-runtime.test.ts did not return in this worktree, so I did not count it as a passing validation result.