Use the owned login WebView for first login too, not just add-account

The Chrome Custom Tab path for the first/only login had real costs -
no way to close it automatically on success, and it's a separate task
outside the app's own navigation - that only bought Chrome's own
autofill in exchange. LoginWebViewView now handles every login, not
just add-account, which was already using it to avoid a Custom Tab
silently reusing Chrome's session for a different account.

url_launcher is no longer a dependency as a result.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

pubspec.lock

@@ -925,30 +925,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.1.0"
-  url_launcher:
-    dependency: "direct main"
-    description:
-      name: url_launcher
-      sha256: f6a7e5c4835bb4e3026a04793a4199ca2d14c739ec378fdfe23fc8075d0439f8
-      url: "https://pub.dev"
-    source: hosted
-    version: "6.3.2"
-  url_launcher_android:
-    dependency: transitive
-    description:
-      name: url_launcher_android
-      sha256: "611e87fb320b70d1dd721dc46af89c98aceccea9b31fde49e084591414e0c610"
-      url: "https://pub.dev"
-    source: hosted
-    version: "6.3.33"
-  url_launcher_ios:
-    dependency: transitive
-    description:
-      name: url_launcher_ios
-      sha256: "8faa1aab294f1ab4040b43660c887b0418d5fa4f0cffef76a484e6aa1092eb4a"
-      url: "https://pub.dev"
-    source: hosted
-    version: "6.4.2"
   url_launcher_linux:
     dependency: transitive
     description:
@@ -957,14 +933,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.2.3"
-  url_launcher_macos:
-    dependency: transitive
-    description:
-      name: url_launcher_macos
-      sha256: "5e835a3b869c2d70325349c81c5a45c28e20791265b67b2669da6b08c5cd5201"
-      url: "https://pub.dev"
-    source: hosted
-    version: "3.2.6"
   url_launcher_platform_interface:
     dependency: transitive
     description: