More "non-code" docking options - esp for Canvas

### Feature summary

Add flexible, non-code-oriented docking and layout options, especially for canvases.

### What problem are you trying to solve?

Canvases currently open in a fixed right-side panel. That works for narrow companion content, but it constrains canvases designed as intelligent dashboards, process cockpits, and process-augmentation screens. These experiences often need horizontal space, persistent visibility, or a layout that complements rather than compresses the main focus area.

The GitHub App is increasingly useful for workflows beyond a traditional code-and-chat arrangement, but its docking model remains optimized primarily for code-centric side panels.

### Proposed solution

Allow users to place canvases and other non-code surfaces in more flexible layouts, including:

- Above or below the main focus area
- Left or right side
- Full-screen
- Detachable or separate-window mode

Placement should be selectable per canvas and remembered across sessions. Resizable split layouts would make the feature especially useful.

### Workflow impact

This would unlock richer process-oriented experiences inside the GitHub App: operational dashboards, planning cockpits, backlog and release-management views, review surfaces, and other agent-augmented workflows. It would let canvas authors design for the task instead of designing around one fixed panel shape.

### Installation context

GitHub Copilot desktop app on Windows.

### Additional context

The key request is not merely moving the existing panel. It is treating canvases as first-class workspace surfaces whose layout can match their purpose, particularly for non-code workflows.

该 issue 未指出文件、测试或实现入口，因此请先定位 Copilot 桌面应用的 canvas 和 docking 布局代码。要视为完成，需要就可选择的放置位置、调整大小、跨会话持久化，以及 Windows 上的可分离或全屏模式达成支持共识。

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