[feat] Add cloud notification modal for macOS desktop users

[deepme987]

Summary

Adds a one-time modal introducing Comfy Cloud to macOS desktop users, with clear messaging that ComfyUI remains free and open source.

Changes

• What: One-time cloud notification modal for macOS desktop users with persistent topbar badge
• Breaking: None
• Dependencies: None

Review Focus

Platform targeting is intentionally macOS + Electron only to minimize backlash and test user reception before broader rollout.

Screenshots (if applicable)

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