Add desktop system notifications for turns / 为桌面端任务完成增加系统通知

[SivanCola]

Summary

• Route desktop controller events through the existing notification sink when [notifications].enabled is set.
• Send system notifications for completed turns, failed turns, approval requests, and ask requests without changing the UI event stream.
• Use the existing macOS/Linux platform senders and upgrade Windows delivery to native Toast notifications through go-toast, relying on its built-in PowerShell fallback.

Related

• Closes [Feature]: 希望桌面端加入通知功能，在跑完的时候通过windows系统发通知 #5294
• Builds on the direction explored in feat(desktop): add Windows Toast notifications via go-toast library (#5294) #5365, but keeps notification enablement tied to the per-workspace loaded config.

Repository contributor credits

• @Barsit in feat(desktop): add Windows Toast notifications via go-toast library (#5294) #5365 proposed the Windows desktop system-notification direction, introduced the go-toast/native Toast approach, and wired desktop controller events into the shared notification sink. This PR adapts that contribution with per-workspace config loading, cross-platform desktop reuse, and focused regression coverage. The adopted feat(desktop): add Windows Toast notifications via go-toast library (#5294) #5365 contribution is also carried in this PR's commit history with a commit-level Co-authored-by trailer for @Barsit.
• @lizhengwu in 为 CLI reasonix chat / reasonix run 增加可选系统通知。默认关闭，用户在配置中开启后，可在任务完成、等待工具批准、等待用户回答时收到系统通知。feat(cli): add desktop notifications #3248 created the shared internal/notify config, sink, and platform sender foundation for CLI notifications. This PR reuses that merged foundation for desktop instead of creating a separate notification path.
• @wade19990814-hue in feat(desktop): add approval and completion notifications #1519 explored desktop approval/completion notifications in the earlier desktop architecture, and @ttmouse in feat: 通知音效系统 — 生成完成 + 等待操作提示音 #4176 added desktop notification sounds for turn completion and attention prompts. Those merged PRs remain prior notification UX work; this PR keeps OS-level system notifications separate from in-app and sound cues.
• @FastTiger1010 in [Feature]: 希望桌面端加入通知功能，在跑完的时候通过windows系统发通知 #5294 reported the desktop background completion/approval notification request that this PR closes.

Cache impact

None. This only wraps local desktop event sinks and platform notification delivery; it does not change provider prompts, tool schemas, serialization, MCP registration, or model-visible context.

Test plan

• go test ./internal/config ./internal/notify
• GOOS=linux GOARCH=amd64 go test -c ./internal/notify -o /tmp/reasonix-notify-linux.test
• GOOS=windows GOARCH=amd64 go test -c ./internal/notify -o /tmp/reasonix-notify-windows.test.exe
• cd desktop && go test .
• go test ./...