fix(secrets): ship the delete modal's own styles under collision-proof class names (0.2.3)

Fixes scadaproj#2. ConfirmDeleteModal used the bare `modal` class and the
Secrets.Ui RCL shipped no CSS at all, silently depending on the host having
no opinion about that selector. Bootstrap 5 does: `.modal { display: none }`
made the delete modal permanently invisible on every Bootstrap host
(ScadaBridge, OtOpcUa, HistorianGateway — confirmed live on the first two)
while every @onclick handler kept working. Only MxGateway, the lone
Bootstrap-free host, could ever have rendered it.

Fix, entirely inside the RCL so a package bump repairs every host with no
host-side changes:
- class vocabulary renamed to zb-secrets-modal / -backdrop / -card / -title
  so no host framework selector can match the elements;
- the component emits its own <style> block alongside the markup. Scoped
  .razor.css was deliberately NOT used: three of the four family hosts never
  link a scoped-CSS bundle, so isolation CSS would silently fail to load —
  the exact defect class being fixed. Theme tokens with fallbacks keep the
  card legible even on a host without the kit stylesheet.
- data-testids unchanged, so existing Playwright gates and bUnit tests keep
  their selectors.

New bUnit pins: no Bootstrap-reserved class name appears in the rendered
markup, and the component ships a style block that both positions the
overlay and gives it a display mode. Suite 182 pass / 0 fail / 0.2.3.

Claude-Session: https://claude.ai/code/session_01BL2Vu1ESDQ9SCN4gVKkdts

ZB.MOM.WW.Secrets/Directory.Build.props

@@ -5,7 +5,7 @@
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <LangVersion>latest</LangVersion>
-    <Version>0.2.2</Version>
+    <Version>0.2.3</Version>
     <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
     <PackageReadmeFile>README.md</PackageReadmeFile>
   </PropertyGroup>