[Feature]: Firmware Update via manager - Update channel

[Export33]

🎯 Feature Description

While you can choose the update channel in the Update via Internet, you can´t do that in the option via manager.

🤔 Problem Statement

It's unclear which version is used.

💡 Proposed Solution

Add Update Channel to Update Source Via Manager.

🔄 Alternatives Considered

No response

📱 Applicable Device Types

No response

🏗️ Feature Area

Device Control & Actions

⭐ Priority Level

Nice to have

⚙️ Technical Considerations

No response

🔍 Duplicate Check

• I have searched for existing feature requests that might be similar to this one

🤝 Contribution

• I would be willing to help implement this feature
• I can provide testing and feedback during development
• I can help with documentation for this feature

[jfmlima]

Implemented in #84. The update dialog now shows the channel selector for the via-manager source too, and the versions listed come from a new endpoint that asks the manager's own index,so they're accurate even when the device has no internet access and can't report anything itself.