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.yaml

@@ -42,7 +42,6 @@ dependencies:
   intl: ^0.19.0
   path: ^1.9.0
   flutter_secure_storage: ^11.1.1
-  url_launcher: ^6.3.1
   dio: ^5.11.1
   path_provider: ^2.1.5
   open_file: ^3.5.10