[FEATURE] Desktop app: show Code sessions from all signed-in accounts on this machine

I use several Claude accounts on one Mac. The desktop app keeps a separate Code session list for each account, so when I switch accounts the conversation I was just in disappears from the sidebar. The transcripts are still on disk and readable, but the app won't list or reopen them under the other account.

What I'd like: an option to see and reopen every local Code session on this machine, whichever account I'm signed into. Or at least a way to hand a session over to another account.

Why it matters: when one account hits its usage limit, I have to switch mid-task and lose my place. Right now this is the main reason I use the terminal instead of the desktop app.

Related: #94507, #88607