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/Program.cs

@@ -162,6 +162,7 @@ builder.Services.AddSingleton<IJobCvMatchService, JobCvMatchService>();
 builder.Services.AddSingleton<ICvAiClassifier, CvAiClassifier>();
 builder.Services.AddSingleton<ICvAiNormalizer, CvAiNormalizer>();
 builder.Services.AddSingleton<IGoogleTokenValidator, GoogleTokenValidator>();
+builder.Services.AddSingleton<IMicrosoftTokenValidator, MicrosoftTokenValidator>();
 builder.Services.AddScoped<IGmailOAuthService, GmailOAuthService>();
 builder.Services.AddSingleton<IGmailJobMatchingService, GmailJobMatchingService>();
 builder.Services.AddSingleton<IGmailCorrespondenceEnrichmentService, NoOpGmailCorrespondenceEnrichmentService>();
@@ -209,6 +210,7 @@ builder.Services.AddScoped<JobImportService>();
 
 var requireAuth = builder.Configuration.GetValue("Auth:Require", false);
 var googleClientId = (builder.Configuration["Auth:GoogleClientId"] ?? "").Trim();
+var microsoftClientId = (builder.Configuration["Auth:MicrosoftClientId"] ?? "").Trim();
 
 var jwtKey = (builder.Configuration["Auth:JwtKey"] ?? "").Trim();
 var ephemeralJwtKey = false;
@@ -234,7 +236,7 @@ builder.Services.AddAuthentication(options =>
     {
         options.ForwardDefaultSelector = ctx =>
         {
-            if (string.IsNullOrWhiteSpace(googleClientId))
+            if (string.IsNullOrWhiteSpace(googleClientId) && string.IsNullOrWhiteSpace(microsoftClientId))
                 return "local";
 
             var auth = ctx.Request.Headers.Authorization.ToString();
@@ -250,9 +252,11 @@ builder.Services.AddAuthentication(options =>
             {
                 var jwt = handler.ReadJwtToken(token);
                 var iss = jwt.Issuer ?? "";
-                return iss is "accounts.google.com" or "https://accounts.google.com"
-                    ? "google"
-                    : "local";
+                if (!string.IsNullOrWhiteSpace(googleClientId) && iss is "accounts.google.com" or "https://accounts.google.com")
+                    return "google";
+                if (!string.IsNullOrWhiteSpace(microsoftClientId) && iss.StartsWith("https://login.microsoftonline.com/", StringComparison.OrdinalIgnoreCase))
+                    return "microsoft";
+                return "local";
             }
             catch
             {
@@ -322,6 +326,23 @@ if (!string.IsNullOrWhiteSpace(googleClientId))
     });
 }
 
+if (!string.IsNullOrWhiteSpace(microsoftClientId))
+{
+    builder.Services.AddAuthentication().AddJwtBearer("microsoft", options =>
+    {
+        // Validate Microsoft (Entra ID / personal account) ID tokens as bearer tokens.
+        // "common" authority + ValidateIssuer=false: multi-tenant issuer varies per tenant id.
+        options.Authority = "https://login.microsoftonline.com/common/v2.0";
+        options.TokenValidationParameters = new TokenValidationParameters
+        {
+            ValidateIssuer = false,
+            ValidateAudience = true,
+            ValidAudience = microsoftClientId,
+            ValidateLifetime = true,
+        };
+    });
+}
+
 builder.Services.AddAuthorization(options =>
 {
     if (requireAuth)