feat(web): native OS notifications for agent events

[Mokson]

Summary

• Fires browser desktop notifications via Web Notification API when agents complete, fail, or need input while the tab is backgrounded
• 2-second debounce collapses multiple near-simultaneous events into a single summary notification ("2 finished, 1 failed")
• Web Audio synthesized chime plays alongside each notification (respects silent: true on the Notification itself)
• Single "Desktop notifications" toggle in General Settings with browser permission request on enable
• Reuses headlineForPhase and buildAgentAwarenessDeepLink from @t3tools/shared/agentAwareness

Inspired by upstream #976.

Changes

• packages/contracts/src/settings.ts: add desktopNotificationsEnabled to ClientSettingsSchema + ClientSettingsPatch
• packages/shared/src/agentAwareness.ts: export headlineForPhase (was private)
• apps/web/src/notifications/osNotification.ts: pure logic (phase resolution, notification formatting, sound synthesis)
• apps/web/src/notifications/OsNotificationCoordinator.tsx: headless coordinator (store subscription, phase diffing, debounced dispatch)
• apps/web/src/components/settings/SettingsPanels.tsx: DesktopNotificationsSettingsRow with permission handling
• apps/web/src/routes/__root.tsx: mount coordinator
• apps/web/src/localApi.test.ts: add new field to test fixtures

Test plan

• Enable "Desktop notifications" in Settings > General, verify browser permission prompt appears
• Background the tab, trigger an agent completion, verify OS notification fires after ~2s
• Trigger multiple agent completions in quick succession, verify they collapse into one summary
• Click the notification, verify window focuses and navigates to the correct thread
• Disable the toggle, verify no more notifications fire
• Deny permission at browser level, verify toggle shows "Blocked" description and is disabled
• Verify notification sound plays (Web Audio chime)