[bug] clicking "New" on an empty conversation creates duplicate empty sessions #246

Summary

On the Chat with AI panel, when the current conversation has no messages,
clicking "New conversation" creates a new empty session on every click
instead of reusing the current one. Empty sessions accumulate in the history
list and in project storage until the 100-conversation limit blocks new
sessions entirely.

Steps to reproduce

1. Open a project and the Chat with AI panel, with the current conversation
   empty.
2. Click "New conversation" several times.
3. Open the history panel: one empty session has been created per click.

Cause

The client generates a fresh conversation ID per click, while the backend's
create path is idempotent only by conversation ID, so the dedupe never
matches: each request archives the previous empty session and creates a new
one. The same accumulation is reachable from the agent/HTTP entry points,
because "reuse the empty active session" is not enforced as a server-side
rule.

Proposed fix

• Frontend: when the current conversation is empty, "New" reuses it instead
  of creating a new session.
• Backend: in the create path, return the current session when it is empty
  instead of archiving it and creating a new one. This protects the agent
  and HTTP entry points as well.

Acceptance

• Repeated clicks with an empty current conversation produce exactly one
  session, and the history list stops accumulating empty entries.
• Backend contract test: repeated create against an empty current session is
  idempotent.