[refactor] Split the CanvasWorkspace mega-component #174

Context

src/canvas/CanvasWorkspace.tsx is 6,761 lines. The core Workspace() is a single function that holds 50 useState, 26 useEffect and 15 useCallback hooks. All canvas state and side effects are concentrated in one place, which makes it hard to unit test and risky to change.

Scope

• Extract sub-components and hooks by responsibility: canvas interaction, selection, generation, history, and import/export.
• Converge state into a reducer/context instead of dozens of scattered useState calls.
• Add behaviour tests before restructuring so behaviour is pinned.

Acceptance criteria

• The file is substantially smaller and Workspace()'s state/effect counts drop below an agreed maintainable threshold.
• Behaviour is unchanged; new canvas unit tests cover the main interactions.
• npm run check:i18n and the existing checks stay green.

Dependencies / notes

• Depends on the test-baseline issue ([test] Add a unit-test baseline for the canvas and rules layers #172).
• Static review baseline: commit a1ff887.