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

[ZhuchkaTriplesix]

Makes the ER diagram (#1063) readable on real schemas.

Layout (erd_layout.dart)

• Layered: a referenced table sits in a layer left of the tables that reference it (longest-path layering, capped so cycles terminate; self references ignored for layering).
• Inside a layer, cards are ordered by the average position of their neighbours (4 barycenter sweeps) to reduce crossings; layers are centred vertically.
• Tables without relations go into a grid below the diagram.
• Wider gaps between layers leave room for edge tracks. withPosition moves one card (drag); the canvas size follows the cards.

Edges (erd_router.dart, new)

• Orthogonal routing with A* over a sparse grid built from the card borders (inflated by 14 px) and the column rows, with a penalty per bend: no edge passes through a table. Each end may leave / enter the left or the right side, whichever is cheaper.
• Overlapping parallel segments of different edges are spread apart (nudging) instead of drawn on top of each other; the segments attached to column rows never move.
• Drawn with rounded corners; crow's foot at the FK side, a bar at the referenced side.

Interaction (erd_view.dart)

• Drag a table to move it; its edges re-route live. The canvas does not pan while a card is dragged; a 4 px slop keeps taps and the double tap that opens the table working.
• Hover highlights the table, its related tables and their edges (accent colour, drawn on top; other edges fade).
• Auto layout restores the computed layout.
• Cards: tinted header with table icon and column count, key / link icons for PK / FK, monospace types, shadow (stronger while dragging). Toolbar uses QueryaActionButton.

Export

SVG uses the on-screen positions and routes (paths with a title per relation).

Tests (test/features/erd/erd_test.dart)

Parents left of children, isolated tables below, no overlap with a cycle and a self reference, withPosition; every route ends on the right card side at the right column row, is orthogonal and never crosses a card; a card placed in the way is avoided; parallel segments of different edges do not overlap; drag moves a card (not the canvas) and Auto layout puts it back. Existing ERD tests unchanged.

Not done: positions are not saved between sessions; very large schemas (hundreds of tables) are routed on every drag move and may feel slow.

Written without running locally; CI is the first run.

Closes #1145