feat(datagrid): fill column with one value across loaded rows (#1304)

[datlechin]

What

Adds a Fill Column action to the data grid. Right-click a column header, choose Fill Column, enter one value (or check Set to NULL), and it sets that value across all loaded rows. Closes #1304.

The change is staged like any other cell edit, so you review it (Preview SQL), Save to apply, or Discard. One Cmd+Z reverts the whole fill.

Scope decision

Fill targets the rows currently loaded in the grid, not every row matching the current filter. This is deliberate:

• The issue scopes it to "the current result set or view."
• No mature native client (TablePlus, DataGrip, Postico, Sequel) offers a whole-table grid fill. They all stay selection/loaded-scoped and staged, to avoid accidental mass updates of unseen rows.
• A whole-table UPDATE ... WHERE <filter> would bypass change-tracking and undo entirely.

Because the fill is staged and reversible before Save, there is no blocking confirmation alert. The dialog states the impact inline ("This sets N loaded rows. Review and Save to apply.").

How it works

• Menu item lives in the column-header context menu, between Copy Column Values and Filter. Built by TableViewCoordinator (the existing NSMenuDelegate), so the whole flow is coordinator-local like Copy Column Values and multi-cell paste.
• Dialog is an NSAlert with an accessory view (text field, plus a "Set to NULL" checkbox on nullable columns), the same pattern as the existing column-rename prompt.
• Apply reuses the multi-cell-paste mechanics: one beginUndoGrouping/endUndoGrouping named "Fill Column", looping commitTypedCellEdit over loaded rows.

Guards

• Hidden on read-only result sets (isEditable).
• Hidden on primary key columns (filling a PK with one value would fail with a duplicate-key error on Save; DataGrip disables it for the same reason).
• No-op when no rows are loaded.
• Already-equal values skip via the existing cell-commit guard.

Unique-constraint columns aren't guarded yet (that metadata isn't reachable synchronously at menu-build time); PK is the worst case and is covered. Noted as a follow-up.

Tests

FillColumnTests covers the decision logic: fills all loaded rows, skips deleted rows, none when read-only, none when empty, NULL vs empty-string resolution, and singular/plural impact text. The apply loop reuses commitTypedCellEdit, which the cell-edit and paste paths already exercise.

Docs / CHANGELOG

• New Fill Column subsection in docs/features/data-grid.mdx.
• Added entry under [Unreleased].

New String(localized:) keys are extracted into Localizable.xcstrings on build.

Not included (deliberate)

• No whole-table / all-filtered-rows server-side UPDATE.
• No keyboard shortcut (contextual action).
• No selection-scoped "Set value" on arbitrary cell selections (separate feature).