New event: auto-link the active team, opponent/competition, quick-add popups, recurring series

- teams is now hard-locked to the active team (a hidden field, not a
  picker) -- there's no "which team" question on a screen already scoped
  to one. groups/club_wide are dropped for the same reason (both widen
  the audience past a single team). invited_members/excluded_members stay,
  re-scoped to sensible pools (add someone off the roster; exclude someone
  on it) instead of "every club member", with a note that a genuinely
  multi-team event still needs the desktop.
- Opponent and competition are now rendered (game-only) -- opponent via a
  standalone picker with its own "+ New" popup, competition via EventForm's
  existing club-agnostic Competition list.
- "+ New location"/"+ New opponent" popups create a Location/Opponent
  scoped to the club without leaving the screen: the modal's own htmx
  request creates the row and hands back the picker pre-selected via an
  out-of-band swap, so the rest of the in-progress form is never touched.
- A "This repeats" toggle switches the same screen to build an EventSeries
  instead of a single Event (frequency/interval/weekdays/duration/until),
  reusing EventSeriesForm and generate_occurrences the same way
  management.views.EventSeriesCreateView does -- including that view's own
  behaviour of not sending a per-occurrence notification (the bulk
  send_deadline_reminders sweep covers it instead, same as a rolling-
  horizon extension).
- Confirmed (and covered with a test) that a single event created this way
  still fires notify_new_event, notifying whoever's freshly invited.

Along the way, found and fixed a real, previously-undetected rendering bug
this surfaced: swapping a multi-choice field's widget to CheckboxSelectMultiple
*after* setting its queryset/choices silently drops what the queryset/choices
setter had already pushed onto the old widget, rendering an empty checkbox
list. Hit this for invited_members/excluded_members and weekdays here, and
found the same pre-existing bug in CoachCreateNewsView's own team checkboxes
(the New Post screen's team picker has been rendering empty) -- fixed all of
them (widget swapped in before queryset/choices, not after), with rendering
(not just queryset) regression tests for each.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

static/css/app.css

@@ -47,6 +47,7 @@
     --radius-md: 0.375rem;
     --radius-lg: 0.5rem;
     --radius-xl: 0.75rem;
+    --radius-2xl: 1rem;
     --ease-out: cubic-bezier(0, 0, 0.2, 1);
     --ease-in-out: cubic-bezier(0.4, 0, 0.2, 1);
     --animate-pulse: pulse 2s cubic-bezier(0.4, 0, 0.6, 1) infinite;
@@ -3234,6 +3235,9 @@
   .z-20 {
     z-index: 20;
   }
+  .z-30 {
+    z-index: 30;
+  }
   .z-50 {
     z-index: 50;
   }
@@ -4495,6 +4499,10 @@
   .rounded-xl {
     border-radius: var(--radius-xl);
   }
+  .rounded-t-2xl {
+    border-top-left-radius: var(--radius-2xl);
+    border-top-right-radius: var(--radius-2xl);
+  }
   .border {
     border-style: var(--tw-border-style);
     border-width: 1px;