Allow canvases to be pinned as first-class items

Right now canvases live inside a session. Once I move to another session or close the panel, the canvas is out of sight and I have to dig back to it. Sometimes a session becomes important precisely because of what its canvas is showing (a dashboard updating, a browser preview, a running terminal), and I want to keep an eye on it even while I work elsewhere.

I'd like to be able to pin a canvas so it becomes a first-class item rather than something buried in a session.

Two placement ideas:

1. Pin it into the left sidebar, alongside sessions, so it sits at the top level and survives switching between sessions.
2. Pin it to the macOS menu bar as a status item (the icons in the top-right of the screen, what AppKit calls an `NSStatusItem` / what SwiftUI calls a `MenuBarExtra`). Clicking the status item would pop the canvas open without bringing the whole app forward.

This is a rough first pass to get the idea down.

未指定实现文件、入口点或测试。首先决定采用侧边栏项目还是 macOS 状态项目，然后检查 session 和 canvas 的表示方式，以及应用当前如何呈现它们。完成的标准是：固定的 canvas 在其 session 之外仍可访问，并且打开时不会将整个应用置于前台。

由索引模型根据 Issue 内容生成。