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.

job-tracker-ui/package.json

@@ -3,6 +3,7 @@
   "version": "0.1.0",
   "private": true,
   "dependencies": {
+    "@azure/msal-browser": "^5.17.0",
     "@emotion/react": "^11.14.0",
     "@emotion/styled": "^11.14.1",
     "@mui/icons-material": "^7.3.9",