feat: add desktop instance connection profiles and active session switching

[ibourgeois]

Context

As a desktop user, I need Katra to manage isolated local and remote instance connections so I can choose which instance I am actively signed into without blending data or auth between them.

Acceptance Criteria

• The desktop app can store multiple instance connection profiles, including local and remote Katra instances.
• Each connection keeps its own isolated auth/session context.
• The desktop app only maintains one active session at a time.
• Switching the active connection changes which instance the client talks to without implicitly merging data across instances.

Notes

This builds on the existing Fortify auth foundation tracked in #33 rather than replacing it. The long-term goal is one shared Katra runtime model across local and remote instances, with the desktop app acting as a client that selects the active instance/session.

Out of Scope

• Cross-instance sync or replication.
• Changing the local vs remote product model beyond connection/session handling.