fix: Prevent accidental message submission during IME composition

Add isComposing check to prevent Enter key from submitting messages
while Japanese (or other) IME input is in progress.

Affected components:
- AssistantChat
- ExpandProjectChat
- SpecCreationChat
- FolderBrowser
- TerminalTabs

ui/src/components/AssistantChat.tsx

@@ -134,7 +134,8 @@ export function AssistantChat({
   }
 
   const handleKeyDown = (e: React.KeyboardEvent<HTMLTextAreaElement>) => {
-    if (e.key === 'Enter' && !e.shiftKey) {
+    // Skip if composing (e.g., Japanese IME input)
+    if (e.key === 'Enter' && !e.shiftKey && !e.nativeEvent.isComposing) {
       e.preventDefault()
       handleSend()
     }