[P2] Consolidate frontend API clients and types

[Gupta91]

Problem

Frontend API code has started to split by domain, but web/src/lib/canvas.ts still re-exports multiple domains for compatibility.

Expected Behavior

Finish migrating imports to domain-specific API clients and types.

Acceptance Criteria

• Scratch, Canvas, workspace, specs, and agent-run API clients have clear ownership.
• Call sites import from domain clients instead of the canvas compatibility module.
• Compatibility exports are removed once no longer needed.