Guard against undefined values in combat log undo payloads

Firestore rejects writes containing undefined values. The pause/resume
and end-combat undo snapshots read encounter fields that may not yet
exist in Firestore, so add ?? false / ?? null / ?? 0 fallbacks to
match the pattern already used in the start-combat undo path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.js

@@ -1692,7 +1692,7 @@ function InitiativeControls({ campaignId, encounter, encounterPath }) {
       logAction(`Combat ${newPausedState ? 'paused' : 'resumed'}: "${encounter.name}"`, { encounterName: encounter.name }, {
         encounterPath,
         updates: {
-          isPaused: encounter.isPaused,
+          isPaused: encounter.isPaused ?? false,
           turnOrderIds: [...(encounter.turnOrderIds || [])],
         },
       });
@@ -1789,10 +1789,10 @@ function InitiativeControls({ campaignId, encounter, encounterPath }) {
       logAction(`Combat ended: "${encounter.name}"`, { encounterName: encounter.name }, {
         encounterPath,
         updates: {
-          isStarted: encounter.isStarted,
-          isPaused: encounter.isPaused,
-          round: encounter.round,
-          currentTurnParticipantId: encounter.currentTurnParticipantId,
+          isStarted: encounter.isStarted ?? false,
+          isPaused: encounter.isPaused ?? false,
+          round: encounter.round ?? 0,
+          currentTurnParticipantId: encounter.currentTurnParticipantId ?? null,
           turnOrderIds: [...(encounter.turnOrderIds || [])],
         },
       });