feat: add interactive pane transfer (drag-detach between workspaces)

[yuloop]

Summary

Enable users to detach a pane from one workspace and attach it to another via mouse interaction, plus a corresponding set of WebSocket API methods for programmatic pane movement.

Proposed Features

Mouse Interaction

• Drag a pane title bar to detach it from its current workspace
• Drop target indicators show available workspaces
• Visual feedback during drag operation

WebSocket API Methods

• pane.preview_transfer — show transfer preview without committing
• pane.commit_transfer — execute the pane transfer
• pane.detach_target_picker — enumerate available detach targets
• pane.make_transfer_detached — detach a pane for reattachment

Motivation

• Currently moving a pane between workspaces requires closing and re-opening the pane
• For users with many workspaces, this disrupts workflow and loses pane state
• Mouse-based drag-and-drop is a natural interaction model that users expect from terminal multiplexers

Notes

This is a feature request only — no PR attached. The implementation involves mouse handling in src/app/input/mouse.rs, runtime mutations in src/app/runtime_mutations.rs, and new API schema entries. Open to discussion on whether this overlaps with or supersedes any in-progress work (e.g. #3411 transfer handoff).

[akbash-bot]

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