Fix explorer back and session usage selection

app/androidApp/src/main/kotlin/com/example/aiapp/Api.kt

@@ -884,6 +884,8 @@ data class UsageWindow(
     val kind: String,
     val label: String,
     val percent: Double,
+    /** Length of this cycle, when the provider reported it. */
+    val durationMinutes: Long?,
     val resetsAt: String?,
     val active: Boolean,
 )
@@ -931,6 +933,10 @@ fun fetchUsage(settings: ServerSettings): List<UsageSnapshot> =
                             kind = window.optString("kind").ifEmpty { "unknown" },
                             label = window.getString("label"),
                             percent = window.getDouble("percent"),
+                            durationMinutes =
+                                if (window.has("durationMinutes")) {
+                                    window.getLong("durationMinutes")
+                                } else null,
                             resetsAt = window.optString("resetsAt").ifEmpty { null },
                             active = window.getBoolean("active"),
                         )