[laurentiu021]

Closes #2809.

WPF's DataGrid binds Delete to ApplicationCommands.Delete and takes the selected rows off its collection whenever it is not read-only and CanUserDeleteRows keeps its default of true. Eight grids are editable for their tick or switch columns and never turned it off, and the implicit DataGrid style does not either: Startup Manager, Uninstaller, Right-click Menu, Environment Variables, Preinstalled Apps, App Updates, Browser Cleaner and Windows Update. Pressing Delete on a selected row made it vanish with no confirmation while nothing changed on the PC: the startup program still ran at boot and the app stayed installed. On Environment Variables, Apply then deleted nothing; on Preinstalled Apps the hidden row stayed ticked, so Remove selected could remove an app no longer on screen. Since each list was added, the oldest in 0.3.0.

• The eight grids set CanUserDeleteRows="False" beside the CanUserAddRows="False" each already sets, as App Blocker, DNS & Hosts, Shortcut Cleaner, File Shredder and New App Alerts do. A read-only grid needs nothing: WPF refuses Delete on one.
• New guard: ArchitectureTests.EveryEditableGrid_TurnsOffTheDeleteKey reads every DataGrid in the views and requires CanUserDeleteRows="False" of each that is not read-only. Its floor is the 35 grids and 11 editable ones measured.

The attribute on each grid rather than a setter in the implicit style, because the five grids that already turned Delete off do it that way, and the guard reads what each grid says.

Docs

README: the keyboard section says Delete does nothing in a list.

Tests

• New StartupViewUiTests.TheDeleteKey_LeavesTheListAlone (the real XAML): with a row selected, Delete cannot run on the Startup list and the list keeps both rows. The view is given only the list it binds: StartupViewModel's scan hands its result to the application's dispatcher once any test has made one, and nothing pumps that dispatcher on the test's STA thread, so a test waiting for the scan hung when it ran after another view test.
• Red before the fix, green after: without the attribute on Startup Manager the guard and the view test both fail, Delete being able to run; without it on Environment Variables the guard fails.
• Run locally, twice: ArchitectureTests (237) and StartupViewUiTests together with the Browser Cleaner, Ping and New App Alerts view tests (13). All four projects build with 0 warnings, and dotnet format --verify-no-changes is clean.