feat(calendar): add explained period feature and update translations

src/i18n/en.json

@@ -143,6 +143,7 @@
 		"uncompletedStages": "Interrupted or unstarted stages",
 		"none": "None",
 		"currentPlacement": "Current explained placement",
+		"explainedPeriod": "Explained time-off period",
 		"outsideHorizon": "Context date outside the planning horizon",
 		"traceTitle": "Advanced: original decision trace",
 		"traceLoading": "The plan is ready; its decision trace is still loading…",