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-lock.json

@@ -8,6 +8,7 @@
       "name": "job-tracker-ui",
       "version": "0.1.0",
       "dependencies": {
+        "@azure/msal-browser": "^5.17.0",
         "@emotion/react": "^11.14.0",
         "@emotion/styled": "^11.14.1",
         "@mui/icons-material": "^7.3.9",
@@ -52,6 +53,27 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/@azure/msal-browser": {
+      "version": "5.17.0",
+      "resolved": "https://registry.npmjs.org/@azure/msal-browser/-/msal-browser-5.17.0.tgz",
+      "integrity": "sha512-/yTnW2TCk9Mh+2b/NOaHAN+MryUNxzRTaJD/YtrqOA9bpBWfTXn/iyReRbaLrK/btBo3stEzLyEvuWp2NZ5DuA==",
+      "license": "MIT",
+      "dependencies": {
+        "@azure/msal-common": "16.11.1"
+      },
+      "engines": {
+        "node": ">=0.8.0"
+      }
+    },
+    "node_modules/@azure/msal-common": {
+      "version": "16.11.1",
+      "resolved": "https://registry.npmjs.org/@azure/msal-common/-/msal-common-16.11.1.tgz",
+      "integrity": "sha512-yPohvMwWLv1XnaWnIUyKUh8CvcVChCGqG/VluGwfGmaAfrZTNt5yQ+sIs462Sgw6+e2K83KGmMJ860p73ZSCrw==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=0.8.0"
+      }
+    },
     "node_modules/@babel/code-frame": {
       "version": "7.29.0",
       "resolved": "https://registry.npmjs.org/@babel/code-frame/-/code-frame-7.29.0.tgz",