feat(timer): allow swapping the secondary source into the main timer slot

The timer view lets operators show an aux timer or the secondary message
as a smaller secondary timer under the main event timer. This adds a
placement control so that selected source can be promoted into the main
slot, swapping positions with the event timer.

The swap is deliberate: the event timer is demoted to the secondary slot
rather than removed, so the show-critical countdown (and its phase colour)
is never lost from screen.

- add `SecondaryPlacement` ('below' | 'main') to the timer message, with
  server validation and a store default
- expose the aux timer direction to the timer view and honour it when
  formatting a promoted aux (previously hard-coded to count-down)
- add a `getTimerSlots` helper that assigns the event timer and secondary
  content to the main/secondary slots, routing phase colour, paused/finished
  styling and font sizing to whichever slot holds the event timer
- add a Placement radio control to the timer view panel (disabled until a
  secondary source is active) and reflect the swap in the control preview

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MbgWVAXTJUX7E5pAmv6Rs7

apps/client/src/features/control/message/TimerPreview.module.scss

@@ -25,6 +25,15 @@
 
 .secondaryContent {
   border-top: 1px solid $white-7;
+  // when the event timer is demoted here (secondary swapped to main) it keeps its colour treatment
+  color: var(--override-colour, inherit);
+
+  &[data-phase='pending'] {
+    color: $ontime-roll;
+  }
+  &[data-phase='overtime'] {
+    color: $playback-negative;
+  }
 }
 
 .blackout {