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/templates/mobile/base.html

@@ -57,7 +57,7 @@
                     {% if club.logo %}
                         <img class="app-crest" src="{{ club.logo.url }}" alt="">
                     {% else %}
-                        <span class="app-crest"></span>
+                        <span class="app-crest app-crest-fallback">{{ club.initials }}</span>
                     {% endif %}
                     <span class="min-w-0">
                         <span class="block truncate font-display text-[19px] leading-none font-extrabold text-white uppercase">{{ club.name }}</span>