feat: add editable connector waypoints

[devkyato]

Problem

Orthogonal and curved connectors choose their route automatically. Complex diagrams would benefit from optional user-controlled waypoints without giving up attached endpoints.

Proposed scope

• Add and drag intermediate waypoints on selected connectors.
• Keep both endpoints attached while nodes move or resize.
• Preserve waypoints in project JSON, undo/redo, copy/paste, and SVG/PNG export.
• Provide a one-click action to reset a connector to automatic routing.

Acceptance criteria

• Waypoint editing works at every zoom level.
• A completed waypoint gesture creates one history entry.
• Export matches the visible route and excludes editing handles.
• Existing projects continue to load without migration errors.