Implement agent auto-update functionality

[Copilot]

Agents had the update infrastructure in place (6-hour ETag-based checks, SignalR-triggered reinstall) but a critical bug prevented it from working correctly, and there was no per-device update trigger in the UI.

Bug fix: ETag never persisted after update detection (UpdaterWin/Linux/Mac.cs)

When a 200 OK response indicated a newer package was available, the agent installed it but never wrote the new ETag to etag.txt. On the next check cycle (or after the fresh agent started), no saved ETag meant the server returned 200 OK again → infinite reinstall loop.

// Now saved before calling InstallLatestVersion()
if (response.Headers.ETag is { } responseEtag)
{
    await File.WriteAllTextAsync("etag.txt", responseEtag.ToString());
}
await InstallLatestVersion();

New: UpdateAgents vs ReinstallAgents (CircuitConnection.cs)

ReinstallAgents sends the update command and deletes the devices from the DB — wrong for an update. Added UpdateAgents which sends ReinstallAgent via SignalR without removing device records.

New: Per-device "Update Agent" button (DeviceCard.razor)

Added an "Update Agent" entry to the DeviceCard Actions dropdown. Disabled when the device is offline. Calls UpdateAgents for the single device and shows a confirmation toast.

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