Change wallet with rollback

[fernandomg]

User story / Problem statement

Changing wallets means connecting while connected.

If the user renounces mid-flight, they should return to the previous live session (not disconnected).

Today connected ignores connect, and leaving connected destroys the session by design.

Expected outcome

Switching wallets is safe to abandon:

• cancel mid-change returns to the previous session intact;
• success swaps sessions atomically.

Acceptance criteria

• connect from connected starts a change attempt without dropping the live session;
• cancel mid-change returns to the previous session intact;
• Success swaps sessions atomically (no window with both or neither).

Technical notes

Blocked by #85. Design decision first; two candidate shapes:

• carry the previous connection through the attempt in context; cancel restores it;
• or let the session state persist while the attempt runs beside it (parallel/child state).