Round the app header crest and show initials without a logo

The header crest was management.css's hexagon clip-path mark, and its
no-logo fallback rendered an empty coloured hexagon with no initials at
all. Both now render as a plain circle, and the fallback shows the
club's initials on its own colour -- same source (Club.logo/Club.initials)
the PWA install icon already uses, just inline in the header instead of
a generated PNG.

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

mobile/tests.py

@@ -51,6 +51,14 @@ class MobileShellTests(TestCase):
         self.assertContains(response, self.club.name)
         self.assertContains(response, 'rel="manifest"')
 
+    def test_header_crest_falls_back_to_round_initials_without_a_logo(self):
+        self.client.force_login(self.user)
+
+        response = self._get("home")
+
+        self.assertContains(response, 'class="app-crest app-crest-fallback"')
+        self.assertContains(response, self.club.initials)
+
     def test_manifest_names_the_club_and_points_at_its_icon(self):
         response = self._get("manifest")