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}" },
                 )
             }

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

@@ -28,7 +28,10 @@ data class AddPairUiState(
     // ── Sync type ────────────────────────────────────────────────────────────
     val syncDirection: SyncDirection = SyncDirection.TWO_WAY,
     val conflictStrategy: ConflictStrategy = ConflictStrategy.KEEP_NEWEST,
-    val deleteBehavior: DeleteBehavior = DeleteBehavior.MIRROR,
+    val deleteBehavior: DeleteBehavior = recommendedDeleteBehavior(SyncDirection.TWO_WAY),
+    // True once the user explicitly picks a deletion behaviour, so changing direction stops
+    // auto-overriding their choice.
+    val deleteBehaviorTouched: Boolean = false,
     val recursive: Boolean = true,
     // ── Schedule ─────────────────────────────────────────────────────────────
     val scheduleType: ScheduleType = ScheduleType.INTERVAL,
@@ -56,6 +59,16 @@ data class AddPairUiState(
     val done: Boolean = false,
 )
 
+/**
+ * Safe default deletion behaviour for a given direction. One-way backups must NOT propagate a
+ * local deletion to the cloud (the whole point of a backup), so they default to KEEP; two-way
+ * sync defaults to MIRROR. The user can always override — all three options stay selectable.
+ */
+internal fun recommendedDeleteBehavior(direction: SyncDirection): DeleteBehavior = when (direction) {
+    SyncDirection.UPLOAD_ONLY, SyncDirection.DOWNLOAD_ONLY -> DeleteBehavior.KEEP
+    SyncDirection.TWO_WAY -> DeleteBehavior.MIRROR
+}
+
 @HiltViewModel
 class AddPairViewModel @Inject constructor(
     private val syncPairDao: SyncPairDao,
@@ -93,6 +106,7 @@ class AddPairViewModel @Inject constructor(
                             syncDirection = pair.syncDirection,
                             conflictStrategy = pair.conflictStrategy,
                             deleteBehavior = pair.deleteBehavior,
+                            deleteBehaviorTouched = true, // preserve the saved choice when editing
                             recursive = pair.recursive,
                             scheduleType = pair.scheduleType,
                             intervalMinutes = pair.scheduleIntervalMinutes,
@@ -119,6 +133,18 @@ class AddPairViewModel @Inject constructor(
 
     fun update(transform: AddPairUiState.() -> AddPairUiState) = _state.update(transform)
 
+    /** Changing direction re-applies the safe deletion default unless the user already chose one. */
+    fun setDirection(direction: SyncDirection) = _state.update { s ->
+        s.copy(
+            syncDirection = direction,
+            deleteBehavior = if (s.deleteBehaviorTouched) s.deleteBehavior else recommendedDeleteBehavior(direction),
+        )
+    }
+
+    fun setDeleteBehavior(behavior: DeleteBehavior) = _state.update {
+        it.copy(deleteBehavior = behavior, deleteBehaviorTouched = true)
+    }
+
     fun save() {
         val s = _state.value
         val errors = buildList {

app/src/test/kotlin/com/syncflow/ui/addpair/RecommendedDeleteBehaviorTest.kt

@@ -0,0 +1,23 @@
+package com.syncflow.ui.addpair
+
+import com.syncflow.domain.model.DeleteBehavior
+import com.syncflow.domain.model.SyncDirection
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+/**
+ * The Add-Pair screen's default deletion behaviour must never wipe a backup. One-way directions
+ * default to KEEP so deleting a file on the phone leaves the cloud copy intact; two-way defaults
+ * to MIRROR. (The user can still override to any of the three options.)
+ */
+class RecommendedDeleteBehaviorTest {
+
+    @Test fun `upload-only defaults to KEEP so backups are never deleted`() =
+        assertEquals(DeleteBehavior.KEEP, recommendedDeleteBehavior(SyncDirection.UPLOAD_ONLY))
+
+    @Test fun `download-only defaults to KEEP`() =
+        assertEquals(DeleteBehavior.KEEP, recommendedDeleteBehavior(SyncDirection.DOWNLOAD_ONLY))
+
+    @Test fun `two-way defaults to MIRROR`() =
+        assertEquals(DeleteBehavior.MIRROR, recommendedDeleteBehavior(SyncDirection.TWO_WAY))
+}