Make the whole page scroll, show referee sign-up on the event page too

The app shell now scrolls as one page (body/document) instead of boxing
content inside an inner overflow-auto <main> under a fixed header/tab bar --
header and tab bar are sticky instead, which reads as a single intuitive
scroll (most noticeable on Calendar's long agenda) and, as a side effect,
means htmx's boosted-navigation scroll-to-top actually resets what the user
sees. The inner "This week"/"Today" sticky sub-headers are no longer sticky
themselves, since they'd otherwise collide with the now-sticky app header.

Event detail also shows the same referee Accept/Decline card the Calendar
row offers, for a signed-in account with a pending or accepted sign-up on
that game -- not just on the Calendar. Both the calendar row's and the
event page's Accept/Decline forms are now htmx-boosted (no hx-boost="false")
since neither is Alpine-toggled, so a response no longer forces a full page
reload.

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

static/css/app.css

@@ -2793,6 +2793,9 @@
   .right-\[3px\] {
     right: 3px;
   }
+  .bottom-0 {
+    bottom: 0;
+  }
   .bottom-full {
     bottom: 100%;
   }