fix(detail): clamp a backwards DTEND instead of dropping the event (#34)

toEventDetailCore returned null when a present DTEND preceded DTSTART, the
only remaining false-drop that surfaces as the generic "Something went
wrong." error screen — the same un-openable trap as the pre-1970 DTSTART
bug, and worse because the user can't even open the malformed event to fix
it. Clamp the end to DTSTART (a zero-length event) instead, matching how
SearchMapper already coerces its end. After this the detail mapper drops a
row only when DTSTART is genuinely absent (unrenderable).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/EventDetailMapperTest.kt

@@ -120,9 +120,14 @@ class EventDetailMapperTest {
     }
 
     @Test
-    fun `dtend before dtstart drops detail`() {
+    fun `dtend before dtstart is clamped to a zero-length event, not dropped`() {
+        // A backwards DTEND is malformed, but dropping it would make the event
+        // un-openable (the "Something went wrong" trap, same as issue #34); keep
+        // it as a zero-length event so the user can still open and fix it.
         val detail = detailReader(dtstart = 2000L, dtend = 1000L).toDetail()
-        assertThat(detail).isNull()
+        assertThat(detail).isNotNull()
+        assertThat(detail!!.instance.start.toEpochMilliseconds()).isEqualTo(2000L)
+        assertThat(detail.instance.end.toEpochMilliseconds()).isEqualTo(2000L)
     }
 
     @Test