refactor: remove redundant Darwin tray stub

[wj-xiao]

📝 Description

Remove the redundant Darwin-specific no-cgo tray stub and rely on the shared no-cgo tray fallback implementation in web/backend/tray_stub_nocgo.go.

This keeps the no-cgo tray behavior in a single place and avoids maintaining duplicate fallback code for Darwin.

Checks were intentionally skipped before opening this PR, per request.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

N/A

📚 Technical Context (Skip for Docs)

• Reference URL: N/A
• Reasoning: web/backend/tray_stub_nocgo.go already covers (darwin || freebsd) && !cgo, so the separate Darwin-only stub is redundant.

🧪 Test Environment

• Hardware: N/A (checks skipped per request)
• OS: macOS (not runtime-tested)
• Model/Provider: N/A
• Channels: N/A

📸 Evidence (Optional)

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☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.