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/Directory.Build.props

@@ -5,7 +5,7 @@
     <Authors>PepperDash Technology</Authors>
     <Company>PepperDash Technology</Company>
     <Product>PepperDash Essentials</Product>
-    <Copyright>Copyright ©  2025</Copyright>
+    <Copyright>Copyright © 2026</Copyright>
     <RepositoryUrl>https://github.com/PepperDash/Essentials</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
     <PackageTags>Crestron; 4series</PackageTags>
@@ -20,4 +20,7 @@
     <None Include="..\..\LICENSE.md" Pack="true" PackagePath=""/>
     <None Include="..\..\README.md" Pack="true" PackagePath=""/>
   </ItemGroup>
+  <ItemGroup>
+    <AssemblyMetadata Include="PackageId" Value="$(PackageId)" />
+  </ItemGroup>
 </Project>