feat: sync activity timer across a player's open sessions

[gaidheal1]

Summary

Fixes #759. If a player has more than one session open (two browser tabs, or a phone + a laptop), starting, labelling, or submitting an activity in one session had no effect on the others — each session's useActivityTimer state was purely local, seeded once from the server and never invalidated by another session's writes.

Changes

Backend

• Add gameplay.utils.broadcast_activity_timer(timer), reusing the existing per-player player_{id} channel group and the send_group_message/{"type": "action"} pattern already used by gameplay/signals.py and utils.control_timers.
• Call it after every timer mutation: start/pause/reset/complete in BaseTimerViewSet, and set_activity/label_activity/complete in ActivityTimerViewSet.
• Also call it from both auto-complete paths in gameplay/tasks.py (the 30s disconnect grace period, and the stale-connection sweep) — these complete a timer server-side without any originating request, so a tab left open elsewhere needs telling too.
• No TimerConsumer changes needed: its existing generic action handler already relays any {"type": "action", ...} group message straight to the socket.

Frontend

• Extend WebSocketActionMessage with the new "activity_timer_update" action and its data.activity_timer payload.
• Wire the previously-stubbed handleGlobalWebSocketEvent dispatcher with an onActivityTimerUpdate option (the load-game stub is left as-is for future use).
• WebSocketContext calls activityTimer.loadFromServer on receipt, mirroring GameContext's existing bootstrap load (same limitSeconds/is_premium resolution). loadFromServer just overwrites local state, so a session echoing its own update back to itself is a harmless no-op.

Testing

• Added/updated backend tests asserting broadcast_activity_timer is called from start/complete/set_activity/label_activity and from both Celery auto-complete tasks.
• User confirmed the full test suite passes locally.

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