feat(chat): persist conversation history with session switching

Add ChatSession and PersistedChatMessage ObjectBox entities and regenerate schema bindings.

Persist user/assistant messages, restore latest session on launch, and add history bottom sheet + New Chat flow.

Harden edge cases with guarded storage reads, one-time persistence warning, duplicate-save guard, and query.remove() cascade deletion.

Author: priyavratuniyal

lib/models/chat_session.dart

@@ -0,0 +1,52 @@
+import 'package:objectbox/objectbox.dart';
+
+/// A persisted chat conversation.
+@Entity()
+class ChatSession {
+  @Id()
+  int id;
+
+  /// Auto-generated from the first user message.
+  String title;
+
+  @Property(type: PropertyType.date)
+  DateTime createdAt;
+
+  @Property(type: PropertyType.date)
+  DateTime lastMessageAt;
+
+  ChatSession({
+    this.id = 0,
+    required this.title,
+    DateTime? createdAt,
+    DateTime? lastMessageAt,
+  }) : createdAt = createdAt ?? DateTime.now(),
+       lastMessageAt = lastMessageAt ?? DateTime.now();
+}
+
+/// A single persisted message belonging to a [ChatSession].
+@Entity()
+class PersistedChatMessage {
+  @Id()
+  int id;
+
+  final session = ToOne<ChatSession>();
+
+  String content;
+
+  /// 0=user, 1=assistant, 2=system (mapped from ChatRole.index).
+  int roleIndex;
+
+  @Property(type: PropertyType.date)
+  DateTime timestamp;
+
+  bool isError;
+
+  PersistedChatMessage({
+    this.id = 0,
+    required this.content,
+    required this.roleIndex,
+    DateTime? timestamp,
+    this.isError = false,
+  }) : timestamp = timestamp ?? DateTime.now();
+}

lib/models/models.dart

@@ -2,4 +2,5 @@ export 'patient.dart';
 export 'lab_report.dart';
 export 'biomarker_result.dart';
 export 'chat_message.dart';
+export 'chat_session.dart';
 export 'model_info.dart';

lib/objectbox-model.json

@@ -194,10 +194,81 @@
         }
       ],
       "relations": []
+    },
+    {
+      "id": "4:542975154161431803",
+      "lastPropertyId": "4:4126270972844958425",
+      "name": "ChatSession",
+      "properties": [
+        {
+          "id": "1:3079507420252501277",
+          "name": "id",
+          "type": 6,
+          "flags": 1
+        },
+        {
+          "id": "2:7755307406885512726",
+          "name": "title",
+          "type": 9
+        },
+        {
+          "id": "3:1585498312990007282",
+          "name": "createdAt",
+          "type": 10
+        },
+        {
+          "id": "4:4126270972844958425",
+          "name": "lastMessageAt",
+          "type": 10
+        }
+      ],
+      "relations": []
+    },
+    {
+      "id": "5:1397208082251365964",
+      "lastPropertyId": "6:5450536964930078917",
+      "name": "PersistedChatMessage",
+      "properties": [
+        {
+          "id": "1:2957032474519806104",
+          "name": "id",
+          "type": 6,
+          "flags": 1
+        },
+        {
+          "id": "2:4019668193248367598",
+          "name": "sessionId",
+          "indexId": "5:2477129363995053950",
+          "type": 11,
+          "flags": 520,
+          "relationTarget": "ChatSession"
+        },
+        {
+          "id": "3:6374967230335097998",
+          "name": "content",
+          "type": 9
+        },
+        {
+          "id": "4:2972012503306002202",
+          "name": "roleIndex",
+          "type": 6
+        },
+        {
+          "id": "5:2296869470696146310",
+          "name": "timestamp",
+          "type": 10
+        },
+        {
+          "id": "6:5450536964930078917",
+          "name": "isError",
+          "type": 1
+        }
+      ],
+      "relations": []
     }
   ],
-  "lastEntityId": "3:3215559289155237862",
-  "lastIndexId": "4:2507639558608191303",
+  "lastEntityId": "5:1397208082251365964",
+  "lastIndexId": "5:2477129363995053950",
   "lastRelationId": "0:0",
   "lastSequenceId": "0:0",
   "modelVersion": 5,