Fix reading history being double counted

Restore groups a backup's history entries by chapter before comparing them with the stored row, so a backup carrying one entry per copy of a duplicated chapter no longer adds its reading time more than once or moves the last read backwards. A removed history entry keeps its 0 instead of becoming NULL, a tracker's start date skips removed history, and the reader clears a session's start time before saving it, so overlapping saves can't count it twice.

app/src/main/java/eu/kanade/domain/track/interactor/AddTracks.kt

@@ -57,9 +57,8 @@ class AddTracks(
 
                 if (track.startDate <= 0) {
                     val firstReadChapterDate = getHistory.await(mangaId)
-                        .sortedBy { it.readAt }
-                        .firstOrNull()
-                        ?.readAt
+                        .mapNotNull { it.readAt?.takeIf { readAt -> readAt.time > 0 } }
+                        .minOrNull()
 
                     firstReadChapterDate?.let {
                         val startDate = firstReadChapterDate.time.convertEpochMillisZone(

app/src/main/java/eu/kanade/tachiyomi/ui/reader/ReaderViewModel.kt

@@ -629,9 +629,9 @@ class ReaderViewModel(
             val chapterId = readerChapter.chapter.id!!
             val endTime = Date()
             val sessionReadDuration = chapterReadStartTime?.let { endTime.time - it } ?: 0
+            chapterReadStartTime = null
 
             upsertHistory.await(HistoryUpdate(chapterId, endTime, sessionReadDuration))
-            chapterReadStartTime = null
         }
     }