Canvas: plugin scaffold, nav item, and pan/zoom surface

[KrasForge]

Summary

Foundation for the Canvas view plugin: register an installable plugin that adds a Canvas nav item and route, and render an empty infinite pan-and-zoom canvas surface. No cards or connections yet.

Part of #342.

Contribution point

View: a full workspace view plus a nav item, provided by an installable plugin (builtin: false, not pre-installed).

Scope

• Register with the plugin runtime as a lazy-loaded builtin: ctx.addView({ id: 'canvas', label: 'Canvas', icon: (a frame or layout icon), order, component }).
• Add a marketplace catalog entry in plugins.ts and a load in catalog.ts with builtin: false.
• Render an infinite canvas surface with pan and zoom, using a view {x,y,k} transform. Reuse the interaction approach from GraphView rather than adding a new dependency.
• Zoom and fit controls consistent with the graph view.

Out of scope

• Note cards, card dragging, connections, and opening notes. Those land in the follow-up sub-issues of Plugin: Canvas view (tracking) #342.

Implementation notes

• Plugin runtime: frontend/src/features/workspace/plugins/.
• Reference interactions (pan, zoom, drag, the view transform): frontend/src/features/workspace/GraphView.tsx.
• Catalog: plugins.ts and catalog.ts.

Acceptance criteria

• Installing the plugin adds a Canvas nav item and route; uninstalling removes both.
• The canvas pans and zooms smoothly; zoom and fit controls work.
• Renders correctly in light and dark themes.