feat(data-grid): cell-level selection and copy

[thomaswasle]

Summary

Introduces two distinct, mutually exclusive selection modes in the data grid:

Cell selection — click any data cell to focus it. The focused cell is highlighted with a blue
inset ring. Ctrl/Cmd+C copies that cell's raw value as a string.

Row selection — click the row-number column on the left to select one or more rows (existing
shift-click range and Ctrl/Cmd-click toggle behaviour is unchanged). Ctrl/Cmd+C copies the
selected rows in the configured format (CSV / JSON).

The two modes clear each other: clicking a cell clears any row selection; clicking a row number
clears cell focus. Clicking the # header (select all) also clears cell focus.

A "Copy Cell" item has been added to the right-click context menu as a discoverable alternative
to the keyboard shortcut.

Changes

• focusedCell state ({ rowIndex, colIndex } | null) tracks the active cell
• copyCellValue(rowIndex, colIndex) formats the value via formatCellValue (handles null, boolean,
  JSON, geometry, BLOB) and writes it to the clipboard
• copyCellFromContext wires the new context menu item to the same logic
• Cell click: sets focusedCell, clears row selection — no longer triggers handleRowClick
• Row-number click: clears focusedCell, then runs handleRowClick as before
• handleSelectAll (the # header): clears focusedCell before selecting all rows
• Focused cell renders with ring-2 ring-inset ring-blue-400 for a clear, layout-neutral visual
  indicator
• Ctrl/Cmd+C handler simplified: focusedCell → copy cell; selectedRowIndices.size > 0 → copy rows
  (the two states are now mutually exclusive)

[debba]

Interesting, I will test it ASAP