feat: add first-run onboarding for local setup or remote connection #94
Description
Context
As a first-time desktop user, I need Katra to guide me into either setting up a local instance or connecting to a remote instance so the app starts with an intentional active session instead of assuming one deployment mode.
Acceptance Criteria
- On first launch, the desktop app prompts the user to choose between local setup and connecting to a remote Katra instance.
- Completing onboarding results in exactly one active instance/session in the client.
- The onboarding flow can be revisited later through settings when adding or switching connections.
Notes
The desktop app should always remain a client for the selected instance. This issue is about the first-run UX and session activation flow, not the deeper auth implementation tracked elsewhere.
Out of Scope
- Full connection settings management after onboarding.
- Remote worker registration and queue execution.