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/appsettings.Development.json

@@ -26,7 +26,8 @@
     "JwtExpiresMinutes": 720,
     "AdminEmail": "admin@example.com",
     "AdminPassword": "CHANGE_ME_STRONG_DEV_PASSWORD",
-    "GoogleClientId": "CHANGE_ME_GOOGLE_CLIENT_ID"
+    "GoogleClientId": "CHANGE_ME_GOOGLE_CLIENT_ID",
+    "MicrosoftClientId": "CHANGE_ME_MICROSOFT_CLIENT_ID"
   },
   "App": {
     "PublicBaseUrl": "https://jobs.cesnimda.uk"