fix(datagrid): put inspector field actions in a context menu for keyboard and VoiceOver (#1490)

[datlechin]

Part of #1490 (keyboard, focus, accessibility). Inspector surface.

The Set NULL / Set DEFAULT / Set EMPTY / copy / SQL-function actions were only reachable through a button that appears on hover, so they were mouse-only. Per HIG, contextual actions belong in a context menu, which is reachable by right-click, the VoiceOver actions rotor, and Full Keyboard Access. This extracts the action list into a shared FieldMenuContent view used by both the existing hover button and a new .contextMenu on each editable field, so the two stay in sync and the resting visual is unchanged.

Native SwiftUI .contextMenu, single source of truth for the actions. Lint clean, style gate clean.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid exposing full text-field actions on picker fields

When kind is .boolean, .enumPicker, or .setPicker, this outer context menu still installs the full FieldMenuContent, including Set EMPTY and SQL Functions, even though those picker editors only model their allowed choices plus NULL/DEFAULT. For example, choosing Set EMPTY or NOW() on a boolean field writes that raw pending value, but BooleanPickerView renders any non-truthy string as false (BooleanPickerView.swift:20-24), so the inspector can show false while saving a different value/function. Please restrict the context menu for picker fields to the same actions their picker menus support, or skip it for pickers.

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