fix: repair popup install binding, dialog titles, presets switch, radio groups, JSON, tests (#4893)

* fix: repair popup install binding, dialog titles, presets switch, radio groups, JSON, tests

- Invoke-WPFInstall: add param block so the right-click app popup binds
  -PackagesToInstall. It previously dropped the argument into $args and
  installed the whole selected list instead of the clicked app (or warned
  that nothing was selected).
- Replace undefined $AppTitle with "WinUtil" in Install/UnInstall warning
  boxes, matching the XAML window title and sibling dialogs.
- tweaks.json: remove stray nested "link" inside the WPFToggleScrollbars
  registry entry (dead property; registry entries expose only the six
  consumed fields).
- Invoke-WPFPresets: fix dead switch cases to the wildcard-pattern
  convention so selectedFeatures/selectedToggles clear correctly.
- Invoke-WPFUIElements: store the group StackPanel in radioButtonGroups so
  grouped radio buttons share one container (else branch was unreachable).
- sanity.Tests.ps1: pass the Windows PowerShell parser script via
  -EncodedCommand; -Command string marshaling corrupted backticks/quotes and
  produced false parse failures.
- install-workflow.Tests.ps1: remove the AppTitle workaround and update
  title assertions to "WinUtil"; drop orphaned cleanup lines.
- Add CHANGES.md with technical and plain-language explanations.

Verified: each bug reproduced against HEAD, fixes demonstrated to change
behavior, full Pester suite 471/471.

* Delete

* test: cover the explicit popup install parameter path

Add a regression test invoking Invoke-WPFInstall -PackagesToInstall with a
package different from the default sync.selectedApps selection and assert
the runspace receives the explicit object, mirroring the Initialize-WPFUI
popup call shape. Existing default-path coverage is unchanged.

pester/sanity.Tests.ps1

@@ -53,7 +53,8 @@ if ($failed.Count -gt 0) {
 }
 '@
 
-            $output = & $windowsPowerShell.Source -NoProfile -NonInteractive -ExecutionPolicy Bypass -Command $parseScript 2>&1
+            $encodedCommand = [Convert]::ToBase64String([System.Text.Encoding]::Unicode.GetBytes($parseScript))
+            $output = & $windowsPowerShell.Source -NoProfile -NonInteractive -ExecutionPolicy Bypass -EncodedCommand $encodedCommand 2>&1
             if ($LASTEXITCODE -ne 0) {
                 throw "Windows PowerShell parser failed:`n$($output | Out-String)"
             }