Make the channel canvas viewable in the chat view (pinned summary, dashboard, briefings), not only inside the management sheet

[NathanielRobertsGPCC]

Problem

The channel canvas is a perfect surface for per-channel living documents, but today it is only reachable by opening the channel management sheet (three-dot "Manage channel" button, then the Canvas row, data-testid="channel-canvas-ingress"). That is three interactions away from the conversation it describes, and it disappears again as soon as the sheet closes.

Issue #3946 shows this is already a discoverability problem ("where i can find canvas in the desktop app?"). This request goes one step further: the canvas should be viewable in or alongside the chat whenever the channel is open, without hunting through the management sheet.

Use cases

These are all real workflows I want today, and the canvas is the right primitive for each; the only thing missing is visibility.

1. Pinned running project summary. A channel per project, with the canvas as a running summary of where the project is up to, pinned and viewable at the top of the channel. Anyone opening the channel sees the current state first, then the conversation.
2. Channel dashboard. A status board maintained by an agent (open issues, current statuses, monitoring highlights, links). The value of a dashboard is glanceability; behind the three-dot menu it stops being glanced at. I run exactly this now (an agent keeps an issues table on the canvas) and the friction of reopening it each visit undermines it.
3. Morning briefings channel. A channel where an agent refreshes the canvas each morning (today's calendar, overnight tickets, anything red in monitoring). Opening the channel should just show the briefing, current as of this morning, with chat underneath for follow-up questions. No taps required.

The common shape: an agent or teammate keeps the canvas current, and the channel's readers should get it pushed into view rather than having to pull it out of a menu.

Proposal

Possible shapes, in rough order of ambition:

1. Canvas toggle in the channel header (next to members), opening the existing canvas view in the right auxiliary pane. The split-pane plumbing already exists: ChannelManagementSheet renders in RightAuxiliaryPane via ChannelManagementAuxiliaryPanel when useSplitAuxiliaryPane is set, and the canvas is already a view inside it.
2. Remember the open state per channel, so a channel whose canvas was opened in the side pane shows it again on next visit (a "pinned" canvas). This is what makes the dashboard and briefing cases work.
3. Collapsed canvas banner at the top of the message list: a one-line preview (the preview text already exists as canvasPreview in ChannelManagementSheet.tsx), expandable in place to the full canvas. This is the best fit for the "running summary at the top" case.

Option 1 alone removes most of the friction; options 2 and 3 make the canvas a real dashboard and briefing surface. A per-channel setting (or per-user preference) for "show canvas on open" would cover the differing needs of a briefing channel versus an ordinary chat channel.

Context

• Desktop: desktop/src/features/channels/ui/ChannelCanvas.tsx, ChannelManagementSheet.tsx (canvas ingress at the canOpenCanvas row), ChannelManagementAuxiliaryPanel.tsx, RightAuxiliaryPane.tsx.
• Related: where i can find canvas in the desktop app? #3946 (canvas discoverability), Proposal: native signed extension panels for channel workspaces #3863 (extension panels for channel workspaces), which could eventually generalise this.