Infinite Canvas — Phase 2: Core panels (Conversation + Terminal)

[Legacynnn]

Part of the Infinite Canvas epic: #61

Phase 2 — Core panels: Conversation + Terminal on the canvas

Make the two most important panel types fully live inside canvas PanelShape hosts.

Scope

• Conversation panel — host src/features/conversation/ (renderer + use-streaming hook) inside a canvas panel. Streaming, sending prompts, scroll-to-bottom must work within a panel that can be moved/resized/zoomed.
• Terminal panel — host the existing PTY terminal (src/features/terminal/ + terminal_commands.rs) inside a canvas panel. Spawn/stream/stdin/interrupt all functional; each terminal keyed by instance UUID.
• Panel chrome — per-panel header with title + controls (close, panel-type label); content stays interactive while panning/zooming.
• Lifecycle — opening a panel binds it to a session/PTY; closing tears down cleanly. Panel ↔ session/PTY mapping persisted in canvas_panels.config.
• Performance baseline — gate/virtualize rendering or input for off-screen panels so multiple live panels don't jank.

Out of scope

Connections between panels (Phase 3), notes/drawing/editor (Phase 4), global controls (Phase 5).

Acceptance

• Multiple conversation and terminal panels run live on one canvas; survive mode toggle + restart (re-attaching to existing sessions/PTYs where applicable).
• Interactions stay smooth with several live panels open.
• Persistence/mapping changes covered by tests; lint/typecheck/test pass.