fix(chat): 会话输入框改为 Enter 发送、Shift+Enter 换行

原先 Enter 换行、Ctrl+Enter 发送，不符合直觉。改为单独 Enter 发送、Shift+Enter
换行，并加 isComposing 保护避免中文输入法选字时误发（与 sessions-tab 一致）。

Claude-Session: https://claude.ai/code/session_01Bs2rrsVHUBTxRusm9espwB

web/src/app/(main)/chat/page.tsx

@@ -77,8 +77,8 @@ function LiveBadge() {
   );
 }
 
-// Composer is the shared bottom input (textarea grows to a cap, Enter newlines,
-// Ctrl/Cmd+Enter sends) — the same affordance across DraftChat and ChatView.
+// Composer is the shared bottom input (textarea grows to a cap, Enter sends,
+// Shift+Enter newlines) — the same affordance across DraftChat and ChatView.
 function Composer({
   value,
   onChange,
@@ -112,11 +112,10 @@ function Composer({
   const fileInputRef = React.useRef<HTMLInputElement>(null);
   const atts = attachments ?? [];
   function onKeyDown(e: React.KeyboardEvent<HTMLTextAreaElement>) {
-    // Enter 换行；只有 Ctrl/Cmd+Enter 才发送(避免长消息误发)。
-    if (e.key === "Enter" && (e.ctrlKey || e.metaKey)) {
-      e.preventDefault();
-      onSend();
-    }
+    // Enter 发送；Shift+Enter 换行。isComposing 时是中文等输入法在选字,不拦截。
+    if (e.key !== "Enter" || e.shiftKey || e.nativeEvent.isComposing) return;
+    e.preventDefault();
+    onSend();
   }
   return (
     <div className="border-t p-3">
@@ -364,7 +363,7 @@ function DraftChat({
         onChange={setInput}
         onSend={send}
         disabled={sending || uploading || !agentKey}
-        placeholder="输入消息，Enter 换行，Ctrl+Enter 发送"
+        placeholder="输入消息，Enter 发送，Shift+Enter 换行"
         leftSlot={agentPicker}
         onPickFiles={pickFiles}
         uploading={uploading}
@@ -692,7 +691,7 @@ function ChatView({
         onChange={setInput}
         onSend={send}
         disabled={running}
-        placeholder={running ? "Agent 正在回复…" : "输入消息，Enter 换行，Ctrl+Enter 发送"}
+        placeholder={running ? "Agent 正在回复…" : "输入消息，Enter 发送，Shift+Enter 换行"}
         running={running}
         onStop={stop}
         stopDisabled={stopping}