Backport PR #1439: packageManifest CWS endpoint + v1/v2 config merge fix

Backports PepperDash/Essentials#1439 (merged to main at 6ceddcf5) onto
dev/v3-routing (v3.0.0-dev-v3-routing.40):

- New GET /packageManifest CWS route reporting Essentials/plugin/touchpanel
  UI package version info (GetPackageManifestRequestHandler).
- EssentialsConfig.Versions now round-trips via [JsonProperty("versions")].
- ConfigReader now detects v1 vs v2 config by presence of system/template
  nodes (previously any versions node caused the v1->v2 merge to be
  skipped), and re-applies the versions node after merge since
  PortalConfigReader.MergeConfigs does not carry it forward.
- Mobile control: track and validate UI client app versions
  (ConnectedClientVersionInfo, MobileControlSystemController changes merged
  cleanly from the PR).
- Directory.Build.props: adds PackageId AssemblyMetadata item (kept this
  branch's 3.0.0-local version).

Conflict resolution notes:
- This branch had already introduced its own AppVersion type (Version +
  RepoUrl only) for TouchpanelWrapperApp/UserInterfaces. Replaced it with
  the PR's unified NugetVersion (adds PackageId/RepoUrl/Name, all
  NullValueHandling.Ignore) to match main's schema, since AppVersion had
  no other references in the codebase.
- ConfigReader.LoadConfig on this branch had already fixed a double
  Stream.ReadToEnd() bug (reading fileContents once); kept that fix and
  layered the PR's v1/v2 detection fix + versions-node preservation on
  top of it, plus this branch's null-ConfigObject-after-merge guard.
- EssentialsWebApi.SetupRoutes: inserted the new packageManifest route
  into this branch's existing (longer) route list, which already
  includes routing-specific endpoints not present on main.

Builds clean (dotnet build on the full solution, 0 errors).

Not pushed - pending local hardware/config testing before push.

src/PepperDash.Essentials.Core/Web/EssentialsWebApi.cs

@@ -90,6 +90,11 @@ public class EssentialsWebApi : EssentialsDevice
                 Name = "ReportVersions",
                 RouteHandler = new ReportVersionsRequestHandler()
             },
+            new HttpCwsRoute("packageManifest")
+            {
+                Name = "GetPackageManifest",
+                RouteHandler = new GetPackageManifestRequestHandler()
+            },
             new HttpCwsRoute("appdebug")
             {
                 Name = "AppDebug",