refactor(coordinator): extract paginateIfPossible helper, add selectedTabAndIndex

[datlechin]

Summary

First focused cleanup pass on MainContentCoordinator. Pagination had seven methods, six of which followed an identical six-line shape:

guard let tabIndex = tabManager.selectedTabIndex,
      tabIndex < tabManager.tabs.count,
      tabManager.tabs[tabIndex].pagination.<condition> else { return }
paginateAfterConfirmation(tabIndex: tabIndex) { pagination in
    pagination.<action>()
}

Each becomes a one-liner via two small extractions.

What's new

• QueryTabManager.selectedTabAndIndex — returns (tab: QueryTab, index: Int)? atomically with the bounds check. Replaces the two-step selectedTabIndex + < tabs.count + tabs[index] pattern. Five lines, single purpose.
• paginateIfPossible(where:mutate:) (private to Pagination) — captures the "active tab + precondition + confirm-discard + mutate" chain. Each public pagination method passes a KeyPath<PaginationState, Bool> (or short closure) for the precondition and a single-statement closure for the mutation.

Result

Pagination methods that used to be 5–6 lines each:

func goToNextPage() {
    paginateIfPossible(where: \.hasNextPage) { $0.goToNextPage() }
}
func goToPreviousPage() {
    paginateIfPossible(where: \.hasPreviousPage) { $0.goToPreviousPage() }
}
func goToLastPage() {
    paginateIfPossible(where: { $0.currentPage != $0.totalPages }) { $0.goToLastPage() }
}

Net −30 LOC in Pagination, +5 LOC in QueryTabManager.

What's not in this PR

There are ~14 other selectedTabIndex + bounds-check sites across MainContentCoordinator+RowOperations, +Filtering, +Navigation, +FKNavigation, etc. Each has slightly different surrounding logic — some use the index for write-back, some use the tab for read-only, some interact with confirmDiscardChangesIfNeeded. Mechanical migration risks subtle behavior changes; each warrants its own focused PR with smoke testing.

selectedTabAndIndex is the foothold for those future PRs.

Test plan

• Build clean
• Manual: Next, Previous, First, Last buttons across multiple pages
• Manual: change page size — list reloads
• Manual: type offset + Go — jumps correctly
• Manual: each action scrolls back to row 0