Actually make the login attempt to the well-known JMAP endpoint

package-lock.json

@@ -16,6 +16,7 @@
         "@types/node": "^16.18.104",
         "@types/react": "^18.3.3",
         "@types/react-dom": "^18.3.0",
+        "base-64": "^1.0.0",
         "bootstrap": "^5.3.3",
         "react": "^18.3.1",
         "react-bootstrap": "^2.10.4",
@@ -25,6 +26,7 @@
         "web-vitals": "^2.1.4"
       },
       "devDependencies": {
+        "@types/base-64": "^1.0.2",
         "prettier": "3.3.3"
       }
     },
@@ -4051,6 +4053,12 @@
         "@babel/types": "^7.20.7"
       }
     },
+    "node_modules/@types/base-64": {
+      "version": "1.0.2",
+      "resolved": "https://registry.npmjs.org/@types/base-64/-/base-64-1.0.2.tgz",
+      "integrity": "sha512-uPgKMmM9fmn7I+Zi6YBqctOye4SlJsHKcisjHIMWpb2YKZRc36GpKyNuQ03JcT+oNXg1m7Uv4wU94EVltn8/cw==",
+      "dev": true
+    },
     "node_modules/@types/body-parser": {
       "version": "1.19.5",
       "resolved": "https://registry.npmjs.org/@types/body-parser/-/body-parser-1.19.5.tgz",
@@ -5540,6 +5548,11 @@
       "resolved": "https://registry.npmjs.org/balanced-match/-/balanced-match-1.0.2.tgz",
       "integrity": "sha512-3oSeUO0TMV67hN1AmbXsK4yaqU7tjiHlbxRDZOpH0KW9+CeX4bRAaX0Anxt0tx2MrpRpWwQaPwIlISEJhYU5Pw=="
     },
+    "node_modules/base-64": {
+      "version": "1.0.0",
+      "resolved": "https://registry.npmjs.org/base-64/-/base-64-1.0.0.tgz",
+      "integrity": "sha512-kwDPIFCGx0NZHog36dj+tHiwP4QMzsZ3AgMViUBKI0+V5n4U0ufTCUMhnQ04diaRI8EX/QcPfql7zlhZ7j4zgg=="
+    },
     "node_modules/batch": {
       "version": "0.6.1",
       "resolved": "https://registry.npmjs.org/batch/-/batch-0.6.1.tgz",