Refactor drag and drop system

[Copilot]

Summary

Addresses the indirection, split implementations, and general annoyances in the drag-and-drop / resize system.

Problem

The old code attached grid geometry functions (validSpot, validResize, getPoint, getRect, resetValid) and the currentOpValid state directly to the Repeater element (id: grid). This caused several problems:

• A Repeater is a view component; using it as a calculation engine was confusing.
• BaseWidget.qml and ResizeComponent.qml both accessed the Repeater by its id (grid) from inside delegate components — the coupling was invisible without knowing the exact scope chain.
• ResizeComponent.qml also used bare model role names (row, column, rowSpan, colSpan) that are only accessible as implicit delegate scope properties, making data flow hard to trace.

Changes

Tab.qml

• Moved all grid logic functions (validSpot, validResize, getPoint, getRect, resetValid) and lastOpSuccessful state from the Repeater to Tab itself.
• Renamed the Repeater id from grid to widgetRepeater (it now only renders widgets — no logic attached).

BaseWidget.qml

• Replaced every grid.* call with tab.*.
• Replaced implicit delegate model-role accesses (row, column, rowSpan, colSpan) with explicit model.row, model.column, model.rowSpan, model.colSpan.

ResizeComponent.qml

• Replaced every grid.* call with tab.*.
• Replaced bare row/column/rowSpan/colSpan delegate-scope accesses with explicit widget.mrow/widget.mcolumn/widget.mrowSpan/widget.mcolumnSpan.

docs/drag-drop-architecture.md (new)

• Documents component responsibilities, key properties/functions, and data-flow diagrams for all three drag scenarios (in-grid drag, from-sidebar drag, resize).

Testing

No automated tests exist for this area; the logic is QML/UI-level. The refactoring is a pure rename/move — no algorithms were changed, only where the functions live and how they are referenced.