Tracer Bullet: Project Chat Multi-Session UX (Create + Switch Sessions) #48

Goal

Validate multi-conversation UX within a single project chat surface before persistence work lands.

Minimal Scope

• Add a sessions sidebar/panel on /app/projects/:projectId/chat.
• Implement create-new-session interaction (in-memory is acceptable for tracer bullet).
• Implement session switching interaction and active-session state.
• Ensure selected session drives which stream/messages are shown.
• Keep to happy path; no deep persistence/error recovery yet.

Acceptance Criteria

• User can create a new chat session in the project chat UI.
• User can switch between sessions in the sidebar/panel.
• Active session selection controls displayed streamed output.
• Document UX/state learnings for persistence phase.

Related Feature

Unlocks: persisted sessions/messages and all-chats view.

Additional Context

This tracer bullet should follow and build on the project chat streaming tracer bullet.