feat(erd): layered layout, orthogonal edges around tables and draggable cards #1145

Problem

The ER diagram (#1063) places tables in a plain grid and draws each relation as one curve between the two cards. With more than a few tables, lines cross cards and each other, and the layout cannot be fixed by hand.

Scope

• Layout: layered (referenced tables to the left, referencing ones to the right), order inside a layer chosen to reduce crossings; tables without relations in a grid below.
• Edges: orthogonal routing around cards (no line through a table), rounded corners, parallel segments spread apart instead of drawn on top of each other; crow's foot at the FK side, bar at the referenced side.
• Drag: tables can be moved with the mouse; edges re-route live; Auto layout restores the computed layout.
• Look: cards with a tinted header, PK / FK icons, hover highlights the table's relations.
• Export: SVG uses the same positions and routes.

Acceptance

• No edge segment passes through a table card (except its own port stubs).
• Dragging a card moves it and re-routes its edges; double tap still opens the table.
• Unit tests for layout (no overlap, parents left of children) and routing (no card crossing, ends on the right column rows).

Follow-up of #1063.