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>

pester/fixtures/legacy-config.json

@@ -0,0 +1,22 @@
+{
+  "Install": [
+    {
+      "winget": "Git.Git",
+      "choco": "git"
+    },
+    {
+      "winget": "Retired.App",
+      "choco": "retired-app"
+    }
+  ],
+  "WPFTweaks": [
+    "WPFTweaksTelemetry"
+  ],
+  "WPFFeature": [
+    "WPFFeatureSandbox"
+  ],
+  "WPFInstall": [
+    "WPFInstallGit",
+    "WPFInstallRetired"
+  ]
+}