fix(edit): stop cursor jumping in event text fields

The event form's state pipeline ran .flowOn(io) over the whole combine,
including the _form round-trip every keystroke depends on. That async hop
handed BasicTextField a lagging value while typing, so Compose kept
correcting the cursor to the stale position.

Scope flowOn(io) to just the calendar/prefs/settings reads and collect the
form -> state -> UI path on the main dispatcher, so keystrokes round-trip
synchronously and the cursor stays put.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/ui/edit/EventEditViewModel.kt

@@ -109,7 +109,7 @@ class EventEditViewModel @Inject constructor(
             prefs.lastUsedCalendarId,
             settingsPrefs.defaultFormFields,
             ::ExternalInputs,
-        ),
+        ).flowOn(io),
     ) { local, external ->
         val form = local.form ?: return@combine null
         val resolvedId = form.calendarId
@@ -131,7 +131,6 @@ class EventEditViewModel @Inject constructor(
                 resolved.rrule != local.editTarget.original.rrule,
         )
     }
-        .flowOn(io)
         .stateIn(
             scope = viewModelScope,
             started = SharingStarted.WhileSubscribed(5_000L),