refactor(datagrid): de-duplicate pagination helpers and guard All rows on unknown totals (#1364)

[datlechin]

Follow-up cleanup to #1408 (the pagination bar, which is now merged). Internal refactors plus one small behavior guard, no change to the public surface.

Refactors (de-duplication)

• PaginationState: extracted a private setPage(_:) that single-sources the currentOffset = (page - 1) * pageSize arithmetic previously repeated across six navigation methods.
• PaginationCoordinator: extracted a private confirmLargeFetch(...) for the warning-alert + sheet/modal presentation that showAllRows and fetchAllRows both duplicated.
• PaginationControlsView: collapsed the two near-identical popovers (jump-to-page, custom page size) into one submitPopover(...) helper.
• MainStatusBarView: extracted isPagedWithUnknownTotal, shared by showsPaginationControls and rowInfoText.

Behavior

• Guard: "All rows…" in the page-size menu is now disabled when the table's total row count is unknown (it previously appeared but silently did nothing there). Folded in here rather than as a separate Fixed entry since the feature is still unreleased.
• Added accessibility labels to the jump and custom page-size text fields.

Everything else is unchanged: same UI, same navigation/disabled logic, same String(localized:) keys, and every navigation method keeps its exact guard and result (the PaginationState tests still assert the same currentPage/currentOffset). SwiftLint --strict is clean.

Not built locally; relying on CI / the Xcode build for the compile.