Canvas: multiple named canvases

[KrasForge]

Summary

Support more than one canvas: create, rename, switch between, and delete named canvases, since layout state is keyed per canvas.

Part of #342. Builds on the scaffold, cards, and connections sub-issues.

Behavior

• Create a new canvas, give it a name, switch between canvases, and delete one.
• Each canvas keeps its own cards, positions, and connections.
• The last-open canvas is restored on reload.

Implementation notes

• Store a list of canvases plus a per-canvas layout record in localStorage; the per-canvas key is already implied by the cards and connections sub-issues.
• A lightweight canvas switcher in the view header, consistent with existing view chrome.

Acceptance criteria

• Canvases can be created, renamed, switched, and deleted.
• Each canvas retains its own layout independently.
• The last-open canvas is restored after a reload.
• Renders correctly in light and dark themes.