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/src/setupTests.ts

@@ -29,12 +29,13 @@ jest.mock('./api', () => ({
 }));
 
 jest.mock('./components/GoogleAuthCard', () => () => null);
+jest.mock('./components/MicrosoftAuthCard', () => () => null);
 
 beforeEach(() => {
   const { api } = require('./api');
   api.get.mockImplementation((url: string) => {
     if (url === '/auth/config') {
-      return Promise.resolve({ data: { requireAuth: false, googleEnabled: false, localEnabled: true, allowRegistration: false } });
+      return Promise.resolve({ data: { requireAuth: false, googleEnabled: false, microsoftEnabled: false, localEnabled: true, allowRegistration: false } });
     }
     if (url === '/auth/me') {
       return Promise.resolve({ data: { roles: [], email: 'demo@example.com', userName: 'demo' } });