Fix merge re-adding deleted/cancelled events and timer sync

- Track removed events via removedSinceLoad set in Logbook to prevent
  merge from re-adding deliberately deleted or cancelled events
- Deduplicate finalized timer events (same type + similar start time)
  to prevent duplicates when both devices stop the same timer
- Detect timer cancellation from other device: dismiss local timer
  dialog when ongoing event disappears from logbook after sync
- Fix thread safety: take snapshot of events before background merge

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

app/src/main/java/it/danieleverducci/lunatracker/entities/Logbook.kt

@@ -5,6 +5,7 @@ class Logbook(val name: String) {
         const val MAX_SAFE_LOGBOOK_SIZE = 30000
     }
     val logs = ArrayList<LunaEvent>()
+    val removedSinceLoad = mutableSetOf<String>()
 
     fun isTooBig(): Boolean {
         return logs.size > MAX_SAFE_LOGBOOK_SIZE