Merge the identity page's App/Website preview tabs and make Email functional

App and Website were always shown together already, so giving them
separate tabs implied a toggle that didn't exist -- merged into one
"App & Website" tab. The Email tab now shows a real, colour-bound
sample (plain header with a secondary-coloured crest badge, a
primary-coloured button) matching the actual branded emails this app
already sends -- audited every mail-sending call site (claims.py,
the new invoicing service, and platform billing's club-facing
reminders) and the first two already use the branded shell; billing's
is intentionally unbranded since it's RosterChief invoicing the club,
not the club invoicing its own members.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

management/tests.py

@@ -4686,6 +4686,19 @@ class ClubSettingsPreviewTests(ManagementTestBase):
         response = self.club_get("club_settings")
         self.assertContains(response, 'name="website"')
 
+    def test_app_and_website_share_one_tab(self):
+        response = self.club_get("club_settings")
+
+        self.assertContains(response, "App & Website")
+        self.assertContains(response, 'data-preview-tab="app"')
+        self.assertNotContains(response, 'data-preview-tab="website"')
+
+    def test_the_email_tab_renders_and_starts_hidden(self):
+        response = self.club_get("club_settings")
+
+        self.assertContains(response, 'data-preview-tab="email"')
+        self.assertContains(response, 'id="preview-panel-email" class="preview-email hidden"')
+
     def test_saving_still_updates_the_club(self):
         response = self.club_post(
             "club_settings",