Add canvas pane size selector (1:1 / 2:1)

[0101]

Problem

When open, the canvas pane could only ever take half the layout. For doc-heavy work the fixed 50/50 split is often too cramped for the canvas, with no way to give it more room.

Changes

Adds a size selector to the canvas header, beside the existing position selector. It offers 1:1 (default, even split) and 2:1 (the open pane takes two-thirds of the layout), and the choice persists in global config.

• New CanvasSize type in Shared/Types.fs (Ratio1To1 | Ratio2To1), threaded through DashboardResponse and IWorktreeApi.
• Persistence in Server/WorktreeApi.fs (readCanvasSize / writeCanvasSize against the canvasSize config key), mirroring how CanvasPosition is stored; DemoFixture.fs and the read-only/fixture APIs updated.
• Client wiring: SetCanvasSize message, CanvasUpdate.setCanvasSize, the CanvasState.CanvasSize field, and 1:1 / 2:1 buttons in CanvasPane.fs (App.fs applies the layout class).
• CSS in index.html: a canvas-size-2to1 rule giving the pane 66.667% flex, reusing the existing position-button styling.
• Spec note added in docs/spec/canvas-pane.md.

The second commit simplifies the feature: it merges the duplicate .canvas-size-group CSS into .canvas-pos-group, and factors the position/size buttons onto one shared toggleButton helper.

Tests

• dotnet build src/Client/Client.fsproj — succeeds, 0 warnings / 0 errors.
• dotnet test src/Tests/Tests.fsproj --filter "Category=Fast" — 959 passed, 0 failed.
• ArchiveTests dashboard JSON fixture updated for the new CanvasSize field.

[Copilot]

Pull request overview

This PR adds a size selector to the canvas pane header, letting users choose between a 1:1 (even split, default) and 2:1 (canvas takes two-thirds of the layout) ratio when the pane is open. It cleanly threads a new CanvasSize domain type through the Shared, Server, and Client layers, persists the choice in global config (mirroring the existing CanvasPosition plumbing), and reflects the choice via a canvas-size-2to1 CSS layout class. A small refactor factors the position and size buttons onto a shared toggleButton helper and merges their CSS selectors.

Changes:

• New CanvasSize DU (Ratio1To1 | Ratio2To1) added to DashboardResponse, IWorktreeApi, and persisted server-side via readCanvasSize/writeCanvasSize.
• Client wiring: SetCanvasSize message, CanvasUpdate.setCanvasSize, CanvasState.CanvasSize, 1:1/2:1 buttons, and a canvas-size-2to1 layout class applied in App.fs.
• CSS, spec docs, and the ArchiveTests JSON fixture updated for the new field.

Show a summary per file

File	Description
src/Shared/Types.fs	Adds CanvasSize DU and threads it into DashboardResponse and IWorktreeApi.
src/Server/WorktreeApi.fs	Adds read/writeCanvasSize persistence and updates both API implementations and getWorktrees.
src/Server/DemoFixture.fs	Sets CanvasSize default on the base dashboard fixture.
src/Client/AppTypes.fs	Adds the SetCanvasSize message.
src/Client/App.fs	Applies size from response on first load and builds the canvas-size-* layout class.
src/Client/CanvasState.fs	Adds CanvasSize field with Ratio1To1 default.
src/Client/CanvasUpdate.fs	Adds setCanvasSize update + persistence command.
src/Client/CanvasView.fs	Wires SetSize callback and passes size to CanvasPane.view.
src/Client/CanvasPane.fs	Adds size buttons and factors a shared toggleButton helper.
src/Client/index.html	Adds canvas-size-2to1 flex rule and merges position/size CSS selectors.
src/Tests/ArchiveTests.fs	Updates dashboard JSON fixture with the CanvasSize field.
docs/spec/canvas-pane.md	Documents the new size selector.

Review details

• Files reviewed: 12/12 changed files
• Comments generated: 1
• Review effort level: Medium