Show sign-in window on first launch when user is not authenticated

**Summary**

When Coder Desktop is launched for the first time (or when the user is not signed in), the app should show the sign-in window instead of silently minimizing to the system tray. Currently, new users see no visible UI after launching the app, which makes it appear broken.

**Problem**

After a fresh install, launching Coder Desktop produces no visible window or taskbar entry. The app goes straight to the system tray as a background process. For a new user who has never used the app before:

1. There is no indication the app launched successfully
2. The user must discover the system tray icon (which Windows often hides behind the ^ overflow arrow)
3. The user must click the tray icon to find the sign-in prompt
4. The app cannot be used at all without signing in, so the silent tray behavior adds unnecessary friction

This leads users to believe the application failed to start — especially since Task Manager shows minimal CPU/memory activity, reinforcing the impression that nothing is happening.

**Proposed Behavior**

• First launch / not signed in: Show the sign-in window automatically. This gives the user a clear next step and confirms the app is running.
• Already authenticated: Continue the current behavior of going silently to the system tray.

Other system tray applications (such as Tailscale, 1Password) all show an onboarding or sign-in window on first launch before settling into tray-only mode.