fix(calendar): one input for the auth code, and a louder warning

Two things reported after testing the flow.

There were two boxes and no way to tell which to use. The config
template's string branch dispatches widgets from an allow-list of names,
and anything missing from it falls through to a plain input type=text --
so the field rendered both the widget's own box and a stray one for the
same key. google-oauth is now on that list, which is all the widget ever
needed to render in place of the fallback rather than beside it.

And the warning that the redirect page fails to load was small grey text
under a link, which is where it is least likely to be read. It is now an
amber callout that leads with "The next page will fail to load. That is
expected." The failure lands at exactly the moment the user has to act
on it, and it looks precisely like the flow breaking rather than
working. The paste box is labelled too, rather than relying on a
placeholder that vanishes on focus.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

test/web_interface/test_calendar_oauth_endpoints.py

@@ -81,6 +81,35 @@ class TestTheRoutesExistAtAll:
         assert "'/plugins/calendar/list-calendars'" in source
         assert "'/plugins/calendar/authenticate'" in source
 
+    def test_the_oauth_widget_is_dispatched_not_rendered_as_a_text_box(self):
+        # The string branch of the config template dispatches on an allow-list
+        # of widget names; anything missing from it silently falls through to a
+        # plain <input type="text">. That produced two boxes on the calendar
+        # page -- the widget's own, and a stray one for the same field -- and
+        # no way to tell which to paste into.
+        template = (Path(project_root)
+                    / 'web_interface/templates/v3/partials/plugin_config.html'
+                    ).read_text(encoding='utf-8')
+        allow_list_line = [ln for ln in template.splitlines()
+                           if "str_widget in [" in ln]
+        assert allow_list_line, "the string widget allow-list moved"
+        assert "'google-oauth'" in allow_list_line[0], allow_list_line[0]
+
+    def test_the_widget_script_is_served(self):
+        base = (Path(project_root) / 'web_interface/templates/v3/base.html'
+                ).read_text(encoding='utf-8')
+        assert 'widgets/google-oauth.js' in base
+
+    def test_the_failed_page_is_called_out_loudly(self):
+        # The loopback redirect lands on a browser error page at exactly the
+        # moment the user has to act. In small grey text it gets missed and the
+        # flow reads as broken while it is working.
+        widget = (Path(project_root)
+                  / 'web_interface/static/v3/js/widgets/google-oauth.js'
+                  ).read_text(encoding='utf-8')
+        assert 'expected' in widget.lower()
+        assert 'amber' in widget, "the warning is not visually distinguished"
+
 
 class TestItSaysWhatIsWrong:
     def test_listing_without_a_token_asks_for_step_2(self, client):