feat(datagrid): add native date/time picker for date columns (#1405)

[datlechin]

Summary

Adds a native date/time picker for DATE, DATETIME, TIMESTAMP, and TIME cells in the data grid. Click the chevron on a date cell to open an NSDatePicker calendar/clock popover and pick a value visually.

The picker augments inline editing, it does not replace it:

• Double-click still opens the inline text editor (type, paste, expressions).
• Right-click Set Value still offers NOW() / CURRENT_TIMESTAMP / CURRENT_DATE / CURRENT_TIME / NULL.
• The chevron opens the picker. Three distinct paths, matching the datagrid convention.

This is the capability removed in #1233. That picker was removed because it blocked text editing and was the only way to edit a date cell. This one sits alongside the inline editor, so it adds the visual picker back without that regression.

How it works

• NSDatePicker, style .clockAndCalendar, mode .single, wrapped in an NSViewRepresentable, shown via the existing PopoverPresenter (.semitransient, like every other cell picker). OK commits; Cancel or click-away commits nothing.
• Components adapt to the column type: DATE shows the calendar only, TIME the clock only, DATETIME / TIMESTAMP both.
• Write-back preserves the value's existing shape. A new DateEditingService decomposes the cell string into date, time, fractional-seconds, and timezone-offset parts, lets the picker edit only the calendar/clock fields, and re-emits the fraction and offset verbatim. Naive types are pinned to UTC so the digits shown match the digits stored (no local-zone drift). NULL or unparseable cells start at the current date/time and write the column's default format.
• ColumnType.isTimeOnly now strips a precision suffix, so TIME(6) is treated as time-only. This also fixes display formatting for those columns.

Files

New:

• DateEditingService.swift for the parse/format round-trip
• DateTimePickerContentView.swift for the SwiftUI popover and NSDatePicker wrapper

Changed: DataGridCellKind (new .date case), DataGridCellView (chevron rendering, hit-test, click), DataGridCellRegistry.resolveKind, DataGridView+Click (chevron dispatch), DataGridView+Popovers (showDateTimePickerPopover), ColumnType.isTimeOnly, CHANGELOG.md, and the stale docs/features/data-grid.mdx section.

Tests

DateEditingServiceTests covers MySQL / ISO / Z / offset round-trips, fractional-second preservation, fraction and offset together, date-only, time-only, the edit-then-format path, NULL / empty / whitespace, unparseable input, and the per-type default format. ColumnTypeTests gains TIME(6), TIMESTAMP(6), and TIMESTAMPTZ precision cases.

Test plan

• Click the chevron on a DATETIME cell. Calendar and clock popover opens at the current value. Pick a date and time, OK. Cell shows the new value, row is marked modified, Save emits the right UPDATE.
• DATE cell shows calendar only; TIME cell shows clock only.
• Double-click a date cell still opens the inline text editor.
• Open the picker, click away without changing anything. Cell is unchanged, row not marked modified.
• A TIMESTAMP value with fractional seconds keeps them after editing.
• A timestamptz value keeps its offset after editing.
• A NULL date cell opens the picker at the current time; OK writes a value in the column's format.
• Right-click Set Value still offers NOW() / CURRENT_TIMESTAMP / NULL.

Closes #1405