feat(datagrid): support column and cell-range selection

[shouwang0527]

Problem

The data grid currently only supports row-level selection (selectedRowIndices: Set<Int>). There is no way to visually select an entire column or a vertical range of cells within a column.

Common scenarios where this is needed:

• Copy a subset of values from one column (e.g., rows 5–20 of an id column) to paste into a WHERE clause or another tool
• Select a column to perform bulk operations (delete column values, batch edit, export just that column)
• Quick comparison: select the same column in two query results side by side

Current workarounds

• Right-click column header → "Copy Column Values" copies the entire column, but there's no visual selection and no way to copy a partial range
• Selecting individual cells one by one isn't possible — only full rows can be multi-selected

Proposed behavior

1. Click column header → select (highlight) the entire column; Cmd+C copies all values
2. Click a cell, then Shift-click another cell in the same column → select the vertical range between them
3. Cmd-click cells → add/remove individual cells to the selection (non-contiguous)
4. Selected cells should have a visual highlight, and Cmd+C should copy the selected values (newline-separated or configurable)

Implementation notes (for context)

The current selection model is row-based (Set<Int> at DataGridView.swift:42). Supporting cell selection would likely require a richer model, e.g., Set<CellIndex> where CellIndex = (row: Int, column: Int), plus updates to:

• SortableHeaderView (header click → select instead of / in addition to sort)
• KeyHandlingTableView (Shift/Cmd-click routing)
• DataGridCoordinator (selection sync)

This is similar to how Excel, DataGrip, and DBeaver handle column/range selection in their data grids.