Add admin-configurable Microsoft/Google external auth

- Add Microsoft and Google OAuth provider support via new NuGet packages
- Store provider credentials/enabled flags in Azure Table Storage
- Add AuthenticationSettingsService and AuthProviderConfig model
- Add AuthenticationSettings singleton for login button visibility
- Add ExternalLoginService for linking external logins to local users
- Conditionally register external auth schemes in Program.cs at startup
- Extend AccountController with external login/callback endpoints
- Update Login.razor to show provider buttons when configured
- Add Authentication tab to AdminHome.razor for runtime config
- Only allow external login for users with pre-existing local accounts
- No DB schema changes; uses existing AspNetUserLogins table
- Add docs/ExternalAuthenticationProvidersPlan.md with full plan and security notes

Author: ADefWebserver

BlazorDataOrchestrator.slnx

@@ -1,6 +1,7 @@
 <Solution>
   <Folder Name="/Solution Items/">
     <File Path="docs/AzureContainerAppsRuntimeFixesPlan.md" />
+    <File Path="docs/ExternalAuthenticationProvidersPlan.md" />
     <File Path="docs/LLMBuildErrorResolutionPlan.md" />
     <File Path="docs/LLMNuGetNuspecAutoConfigPlan.md" />
   </Folder>