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/Services/StartupInitializationExtensions.cs

@@ -241,6 +241,9 @@ public static class StartupInitializationExtensions
             `GoogleSubject` longtext NULL,
             `GoogleEmail` longtext NULL,
             `GoogleLinkedAt` datetime(6) NULL,
+            `MicrosoftSubject` longtext NULL,
+            `MicrosoftEmail` longtext NULL,
+            `MicrosoftLinkedAt` datetime(6) NULL,
             PRIMARY KEY (`Id`)
         ) CHARACTER SET=utf8mb4;
 
@@ -353,7 +356,10 @@ public static class StartupInitializationExtensions
             "AvatarImageDataUrl" TEXT NULL,
             "GoogleSubject" TEXT NULL,
             "GoogleEmail" TEXT NULL,
-            "GoogleLinkedAt" TEXT NULL
+            "GoogleLinkedAt" TEXT NULL,
+            "MicrosoftSubject" TEXT NULL,
+            "MicrosoftEmail" TEXT NULL,
+            "MicrosoftLinkedAt" TEXT NULL
         );
         """);
 
@@ -431,6 +437,9 @@ public static class StartupInitializationExtensions
                 EnsureColumn(conn, "AspNetUsers", "GoogleSubject", "ALTER TABLE AspNetUsers ADD COLUMN GoogleSubject TEXT NULL;");
                 EnsureColumn(conn, "AspNetUsers", "GoogleEmail", "ALTER TABLE AspNetUsers ADD COLUMN GoogleEmail TEXT NULL;");
                 EnsureColumn(conn, "AspNetUsers", "GoogleLinkedAt", "ALTER TABLE AspNetUsers ADD COLUMN GoogleLinkedAt TEXT NULL;");
+                EnsureColumn(conn, "AspNetUsers", "MicrosoftSubject", "ALTER TABLE AspNetUsers ADD COLUMN MicrosoftSubject TEXT NULL;");
+                EnsureColumn(conn, "AspNetUsers", "MicrosoftEmail", "ALTER TABLE AspNetUsers ADD COLUMN MicrosoftEmail TEXT NULL;");
+                EnsureColumn(conn, "AspNetUsers", "MicrosoftLinkedAt", "ALTER TABLE AspNetUsers ADD COLUMN MicrosoftLinkedAt TEXT NULL;");
 
                 static void EnsureUserRuleSettingsTable(DbConnection c)
                 {
@@ -757,6 +766,9 @@ public static class StartupInitializationExtensions
                     EnsureMySqlColumn(conn, "AspNetUsers", "GoogleSubject", "ALTER TABLE `AspNetUsers` ADD COLUMN `GoogleSubject` longtext NULL;");
                     EnsureMySqlColumn(conn, "AspNetUsers", "GoogleEmail", "ALTER TABLE `AspNetUsers` ADD COLUMN `GoogleEmail` longtext NULL;");
                     EnsureMySqlColumn(conn, "AspNetUsers", "GoogleLinkedAt", "ALTER TABLE `AspNetUsers` ADD COLUMN `GoogleLinkedAt` datetime NULL;");
+                    EnsureMySqlColumn(conn, "AspNetUsers", "MicrosoftSubject", "ALTER TABLE `AspNetUsers` ADD COLUMN `MicrosoftSubject` longtext NULL;");
+                    EnsureMySqlColumn(conn, "AspNetUsers", "MicrosoftEmail", "ALTER TABLE `AspNetUsers` ADD COLUMN `MicrosoftEmail` longtext NULL;");
+                    EnsureMySqlColumn(conn, "AspNetUsers", "MicrosoftLinkedAt", "ALTER TABLE `AspNetUsers` ADD COLUMN `MicrosoftLinkedAt` datetime NULL;");
 
                     if (!HasMySqlTable(conn, "RuleSettings"))
                     {