[codex] add configurable notification system

[DorianZheng]

What changed

• added a main-process notification system under src/electron/main/notifications/ with shared types, persisted settings, delivery history, throttling, do-not-disturb handling, macOS delivery, Telegram delivery, and idle-agent polling
• wired notification IPC into main.ts, preload, and the shared desktop bridge so renderer settings can load, update, and clear notification state
• hooked runtime events so item transitions into review and acceptance emit notifications, and scheduled task failures surface agent-error notifications
• added a renderer Notifications settings panel with trigger toggles, delivery channel controls, Telegram chat id input, quiet hours, and notification history

Why

Dune did not have a host-level notification system for workflow milestones or runtime failures. This adds a central notification manager so delivery policy, throttling, quiet hours, and channel fan-out live in one place instead of being scattered across the runtime and UI.

Impact

• users can enable or disable specific notification triggers and channels from Settings
• macOS notifications work from the Electron main process
• Telegram notifications can be mirrored to a configured chat when a Telegram bridge is available
• notification history is visible and clearable from Settings
• item review/acceptance transitions, agent task failures, and optional idle-agent alerts now surface outside the app window

Validation

• pnpm run typecheck
• pnpm vitest run src/electron/preload.test.ts src/renderer/features/settings/config/settings-sections.test.ts was attempted but failed in this worktree because @vitejs/plugin-react is not installed in the local test environment