feat(desktop): add tiled session panes

[IchenDEV]

Summary

• add resizable horizontal and vertical session panes with split, focus, and close controls
• keep session, transcript, composer state, editor revision, and scroll behavior isolated per pane
• route asynchronous session creation, transcript loads, prompt acceptance, queue, and steer events back to their originating pane
• prevent duplicate bindings by focusing the pane that already owns a selected session

Validation

• bun test — 719 passed
• bun run build:renderer
• rebased onto the latest main