Fix legacy json import compatibility, UI sidebar refactor, and improved test coverage (#4911)

* Fix backward compatibility for old-style JSON config imports

* Refactor sidebar UI generation and add tests for Get-WinUtilVariables

* Fix appnavigation config test following UI generation refactor

* Restore global sync state in variables tests

* fix(impex): migrate supported legacy selections

Keep modern imports strict while allowing legacy backups to skip retired entries with clear logging and user feedback. Add fixtures covering partial and all-retired imports.

* test: strengthen UI and global state coverage

Exercise app category rendering through Initialize-WPFUI and restore global sync without leaking test state.

* fix(impex): ignore legacy metadata fields

Limit legacy selection migration to supported WPF key families so unrelated string metadata does not produce false retired-setting warnings.

* docs: clarify legacy config imports

Distinguish strict modern flat imports from partial legacy object migration, including single-setting export shape and historical groups.

---------

Co-authored-by: Chris Titus <contact@christitus.com>

functions/private/Update-WinUtilSelections.ps1

@@ -3,7 +3,9 @@ function Update-WinUtilSelections {
         [Parameter(Mandatory)]
         [string[]]$flatJson,
 
-        [switch]$Replace
+        [switch]$Replace,
+
+        [switch]$SkipUnknown
     )
 
     $nextSelections = @{
@@ -25,6 +27,10 @@ function Update-WinUtilSelections {
         }
 
         if (-not $listName) {
+            if ($SkipUnknown) {
+                $cbkey
+                continue
+            }
             throw "Unsupported selection key '$cbkey'."
         }
 
@@ -47,12 +53,21 @@ function Update-WinUtilSelections {
         }
 
         if (-not $isKnownSelection) {
+            if ($SkipUnknown) {
+                $cbkey
+                continue
+            }
             throw "Unknown selection key '$cbkey'."
         }
 
         $nextSelections[$listName].Add($cbkey)
     }
 
+    $validSelectionCount = ($nextSelections.Values | ForEach-Object { $_.Count } | Measure-Object -Sum).Sum
+    if ($SkipUnknown -and $validSelectionCount -eq 0) {
+        return
+    }
+
     if ($Replace) {
         foreach ($listName in $nextSelections.Keys) {
             $sync[$listName] = $nextSelections[$listName]