Direction-aware default for deletion behaviour (don't wipe backups)

The Add-Pair screen defaulted deleteBehavior to MIRROR for every direction,
so an Upload-only backup would delete cloud files when you deleted them on
the phone. Now the default follows the direction:
- Upload-only / Download-only -> KEEP (deleting locally leaves the cloud copy)
- Two-way -> MIRROR
All three options remain selectable; once the user explicitly picks one,
changing direction no longer overrides it, and editing a saved pair keeps
its stored choice. Adds RecommendedDeleteBehaviorTest.

app/src/main/kotlin/com/syncflow/ui/addpair/AddPairScreen.kt

@@ -163,7 +163,7 @@ fun AddPairScreen(onDone: () -> Unit, vm: AddPairViewModel = hiltViewModel()) {
                     label = "Direction",
                     options = SyncDirection.entries,
                     selected = s.syncDirection,
-                    onSelect = { vm.update { copy(syncDirection = it) } },
+                    onSelect = { vm.setDirection(it) },
                     itemLabel = { "${it.label}  —  ${it.description}" },
                 )
                 Spacer(Modifier.height(8.dp))
@@ -179,7 +179,7 @@ fun AddPairScreen(onDone: () -> Unit, vm: AddPairViewModel = hiltViewModel()) {
                     label = "Deletion behaviour",
                     options = DeleteBehavior.entries,
                     selected = s.deleteBehavior,
-                    onSelect = { vm.update { copy(deleteBehavior = it) } },
+                    onSelect = { vm.setDeleteBehavior(it) },
                     itemLabel = { "${it.label}  —  ${it.description}" },
                 )
             }