[P1] Tighten window typings and state boundaries in WorkspaceCanvas

[nkanf-dev]

Summary

Replace permissive any usage around workspace windows with typed interfaces and clearer state boundaries.

Why

WorkspaceCanvas drives high-value interaction logic. Weak typing here increases the risk of subtle state bugs during drag, resize, docking, and activation flows.

Current evidence

• src/components/workspace/WorkspaceCanvas.tsx uses any[] for windows and any for update payloads.

Scope

• Use ReaderWindow-based props and update payloads.
• Make drag/resize helpers type-safe.
• Clarify which fields are mutable in update callbacks.
• Avoid broad partial updates where a narrower command model would be safer.

Acceptance criteria

• WorkspaceCanvas no longer exposes any in its public props.
• Window interaction logic compiles with strong types.
• Refactors in window state become safer and easier to review.

Out of scope

• A full rewrite of the workspace interaction model.