fix: tighten chat selection state invariants

android/app/src/main/java/com/hyzq/boss/ProjectChatUiState.java

@@ -2,6 +2,7 @@ package com.hyzq.boss;
 
 import androidx.annotation.Nullable;
 
+import java.util.Collections;
 import java.util.LinkedHashSet;
 import java.util.Set;
 
@@ -12,9 +13,10 @@ public final class ProjectChatUiState {
         public final boolean multiSelecting;
         public final Set<String> selectedMessageIds;
 
-        public SelectionState(boolean multiSelecting, Set<String> selectedMessageIds) {
-            this.multiSelecting = multiSelecting;
-            this.selectedMessageIds = new LinkedHashSet<>(selectedMessageIds);
+        private SelectionState(Set<String> selectedMessageIds) {
+            LinkedHashSet<String> normalizedIds = new LinkedHashSet<>(selectedMessageIds);
+            this.multiSelecting = !normalizedIds.isEmpty();
+            this.selectedMessageIds = Collections.unmodifiableSet(normalizedIds);
         }
     }
 
@@ -27,21 +29,24 @@ public final class ProjectChatUiState {
     }
 
     public static SelectionState emptySelection() {
-        return new SelectionState(false, new LinkedHashSet<>());
+        return new SelectionState(new LinkedHashSet<>());
     }
 
     public static SelectionState toggleSelection(@Nullable SelectionState current, String messageId) {
+        if (messageId == null || messageId.trim().isEmpty()) {
+            throw new IllegalArgumentException("messageId must not be blank");
+        }
         SelectionState state = current == null ? emptySelection() : current;
         LinkedHashSet<String> selectedMessageIds = new LinkedHashSet<>(state.selectedMessageIds);
         if (selectedMessageIds.contains(messageId)) {
             selectedMessageIds.remove(messageId);
-            return new SelectionState(!selectedMessageIds.isEmpty(), selectedMessageIds);
+            return new SelectionState(selectedMessageIds);
         }
         selectedMessageIds.add(messageId);
-        return new SelectionState(true, selectedMessageIds);
+        return new SelectionState(selectedMessageIds);
     }
 
     public static boolean canForwardSelection(@Nullable SelectionState state) {
-        return state != null && state.selectedMessageIds.size() >= 2;
+        return state != null && state.multiSelecting && state.selectedMessageIds.size() >= 2;
     }
 }