Collapses the ~10 per-modal Show*Modal Func callbacks (wired separately on the shell and the lists island) into one IDialogService + WindowDialogService impl. Removes the RepoImport/WorktreesOverview dialog construction duplicated across MainWindow and ListsIslandView, plus the Confirm/Error dialogs duplicated in both code-behinds. Shell/lists Open* commands now route through an injected Dialogs handle (propagated shell->lists); the per-list worktrees overview also wires conflict resolution now, matching the global one. No VM ctor changes (Dialogs is a settable seam), so no test-fake impact.
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