Cluster 1 — code quality + dialog XamlRoot fix + CSV importer hardening [v2.0]

[pexatar]

First cluster of the PassKey 2.0 refactor described in
piano-revisione-correzione-miglioramento.md (private notes file).

Summary

Code quality / modernization

• Extract BaseDetailViewModel<TEntry> with a template-method pattern; the four Detail VMs (Password, CreditCard, Identity, SecureNote) inherit shared add/edit/save/delete plumbing — ~200 LOC of duplication removed.
• Introduce IVaultEntry interface as the generic constraint.
• Replace Task.Run(async () => await …) in LoginVM/SetupVM with Task.Run(Func<Task>) to keep KDF/decrypt off the UI thread without the nested async-over-async wrapper.
• Implement IDisposable on MainVM and the four List VMs; named handlers replace closures so VaultLocked / NavigationStack.CurrentChanged can be unsubscribed, and Entries collections are cleared on vault lock to avoid retaining decrypted data after lock.
• Extract BIN prefixes in CardTypeDetector to a dedicated BinPrefixes static class.

Dialog / UX fixes

• Pre-existing bug fix: ContentDialog.ShowAsync threw "This element does not have a XamlRoot" on every delete confirmation triggered from a ViewModel. New IDialogQueueService.ConfirmAsync + lazy XamlRootAccessor delegate resolves Window.Content.XamlRoot just-in-time on the UI thread; the duplicated factory pattern is removed from 5 call sites and the crash is fixed.
• Pre-existing UX bug fix: importing an empty / unrecognised file silently advanced to the merge dialog and then showed "Import completed" with zero changes. SettingsViewModel.ImportAsync now surfaces an explicit error when the parsed vault is empty.

CSV importer — NordPass support

• Accept note (singular) as alias for the Notes column header.
• Detect a type column and skip non-password rows so card / identity / note labels do not pollute the imported password list (richer cross-type import will land in a later cluster).
• Two new unit tests cover the NordPass-shape header and the singular-note alias.

Test plan

• dotnet test → 178 / 178 (was 176; +2 NordPass cases)
• Local build via scripts/build-installer.ps1 produces both PassKey-Setup-x64.exe and PassKey-Portable-x64.zip
• User-facing smoke test: delete confirmation works on all 4 entry types; NordPass CSV with 238 password rows imports cleanly; lock/unlock + add/edit cycles show no regressions