fix(desktop): 应用图标 PNG 转 8-bit RGBA 修复 macOS 打包

[DeliciousBuding]

Summary

• tauri-bundler 生成 icns 时遇 16-bit RGBA PNG 报 unsupported ColorType: Rgba16（仅 macOS 路径触发，Windows NSIS 用 .ico 不受影响）
• 修复：四个 agenthub-icon-rounded-*.png 转 8-bit RGBA（IHDR bitdepth=8 校验通过）

Test plan

• CI 全绿
• rc.5 重跑时 macOS Build Tauri (DMG) 通过

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⛔ Files ignored due to path filters (4)

• app/desktop/src-tauri/icons/agenthub-icon-rounded-128.png is excluded by !**/*.png
• app/desktop/src-tauri/icons/agenthub-icon-rounded-256.png is excluded by !**/*.png
• app/desktop/src-tauri/icons/agenthub-icon-rounded-32.png is excluded by !**/*.png
• app/desktop/src-tauri/icons/agenthub-icon-rounded-512.png is excluded by !**/*.png

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