feat(auth): Microsoft OAuth sign-in/link + self-serve signup via Google/Microsoft

Wave 7. Mirrors the existing Google ID-token-exchange pattern (Program.cs
smart-scheme dispatch, JWT bearer scheme, AuthController exchange/link/
unlink endpoints, ApplicationUser fields, reconciler columns) for
Microsoft Entra ID + personal accounts via the multi-tenant "common"
endpoint.

Google/Microsoft sign-in previously only worked for accounts already
linked to an existing local user -- there was no way to actually sign
up via OAuth. Both exchange endpoints now create a new user when no
match is found and Auth:AllowRegistration is true, same gate as
email/password registration.

Frontend: new MicrosoftAuthCard (MSAL popup flow -- Microsoft has no
vanilla-JS equivalent to Google's Identity Services script) wired into
the login page's provider tabs and the profile page's account-linking
section. REACT_APP_MICROSOFT_CLIENT_ID env var, Auth:MicrosoftClientId
config gate on the backend.

JobTrackerApi.Tests/ClientErrorsControllerTests.cs

@@ -51,7 +51,7 @@ public sealed class ClientErrorsControllerTests
         var userManager = TestHostFactory.CreateUserManager();
         userManager.Setup(x => x.GetUserAsync(It.IsAny<ClaimsPrincipal>())).ReturnsAsync(user);
 
-        var controller = new AuthController(BuildConfig(), userManager.Object, Mock.Of<ITokenService>(), Mock.Of<IAppEmailSender>(), Mock.Of<IGoogleTokenValidator>(), Mock.Of<ILogger<AuthController>>())
+        var controller = new AuthController(BuildConfig(), userManager.Object, Mock.Of<ITokenService>(), Mock.Of<IAppEmailSender>(), Mock.Of<IGoogleTokenValidator>(), Mock.Of<IMicrosoftTokenValidator>(), Mock.Of<ILogger<AuthController>>())
         {
             ControllerContext = new ControllerContext
             {