feat: Disable node dragging and conditionally display Source section

[lornakelly]

Description

Disable node and diagram dragging on the canvas at all times - whether readonly is true of false. Node positioning is owned entirely by the layout engine.
The side-panel Source section is shown only when isReadOnly is true. When isReadOnly is false (editable mode) the source section should be hidden.

Motivation

• The editor is moving to dynamic/auto layout rather than drag-and-drop positioning
• Source section is a read-only reference, hidden in edit mode. In editable mode the structured fields are the authoring surface; a parallel source view sitting next to them is a competing second surface that invites confusion about which one you're editing

Proposed Implementation

No response

Definition of Done

• Implementation: Fully implemented according to the Open Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.