refactor(erd): decompose 2900-line ErdView into pure ErdLayoutEngine, ErdCanvas, ErdTableCard and ErdMinimap #1362

Summary

lib/features/erd/erd_view.dart is currently the largest file in the codebase at 2,895 lines.
It violates the Single Responsibility Principle by combining multiple major subsystems:

1. Algorithmic Graph Layout: Dagre/force-directed positioning, rank assignment, layer assignment, FK edge routing and collision resolution.
2. Interactive Canvas: Pan/zoom gesture handling (InteractiveViewer / GestureDetector), scale clamping, drag selection.
3. Table Node Rendering: Node cards, column rows, PK/FK badges, data type labels, table context menus.
4. Minimap & Navigation: Miniature viewport representation and viewport jumping.
5. Image Export: PNG/SVG rendering and file export flows.

This makes modifying ERD features risky and hinders testing graph layout algorithms in isolation without mounting heavy widget trees (related to Epic #1286 and Epic #1166).

Proposed Solution

Decompose erd_view.dart into dedicated single-responsibility modules:

1. lib/core/erd/erd_layout_engine.dart: Pure non-widget algorithmic node placement and edge routing class, 100% unit-testable.
2. lib/features/erd/widgets/erd_canvas.dart: Canvas gesture handling, zoom/pan transform matrix, and edge painter.
3. lib/features/erd/widgets/erd_table_card.dart & erd_column_row.dart: Table card rendering, selection states, and badges.
4. lib/features/erd/widgets/erd_minimap.dart: Isolated minimap widget.
5. lib/features/erd/controllers/erd_view_controller.dart: State management for table selection, focus, and search filters.

Acceptance Criteria

• erd_view.dart acts purely as a coordinator under 300 lines.
• Graph layout logic is covered by pure Dart unit tests without Flutter widget tester.
• Existing ERD behavior, interaction, and visual tests remain green.