perf: collapse long session messages to reduce text layout cost

Messages over 3000 characters are now truncated to 1500 characters by
default, with an expand/collapse toggle. This avoids expensive browser
text layout for large AI responses containing code or tool output.

src/i18n/locales/en.json

@@ -725,7 +725,9 @@
     "copyCommand": "Copy Command",
     "copyMessage": "Copy Message",
     "messageCopied": "Message copied",
-    "conversationHistory": "Conversation History"
+    "conversationHistory": "Conversation History",
+    "expandContent": "Expand full content",
+    "collapseContent": "Collapse"
   },
   "console": {
     "providerSwitchReceived": "Received provider switch event:",

src/i18n/locales/ja.json

@@ -725,7 +725,9 @@
     "copyCommand": "コマンドをコピー",
     "copyMessage": "メッセージをコピー",
     "messageCopied": "メッセージがコピーされました",
-    "conversationHistory": "会話履歴"
+    "conversationHistory": "会話履歴",
+    "expandContent": "全文を表示",
+    "collapseContent": "折りたたむ"
   },
   "console": {
     "providerSwitchReceived": "プロバイダー切り替えイベントを受信:",

src/i18n/locales/zh.json

@@ -725,7 +725,9 @@
     "copyCommand": "复制命令",
     "copyMessage": "复制消息",
     "messageCopied": "已复制消息内容",
-    "conversationHistory": "对话记录"
+    "conversationHistory": "对话记录",
+    "expandContent": "展开完整内容",
+    "collapseContent": "收起"
   },
   "console": {
     "providerSwitchReceived": "收到供应商切换事件:",