fix: prevent form submission during IME composition (#1069)

## Problem
When typing in Chinese (or other IME-based languages like
Japanese/Korean), pressing Enter to select a character from the IME
candidate list would incorrectly submit the message instead of
confirming the character selection.

## Solution
Added IME composition state detection in the `handleKeydown` function in
`ChatForm.svelte`:
- Check `event.isComposing` to detect active IME composition
- Fallback to `event.keyCode === 229` for broader browser compatibility
- Return early when IME is active, allowing normal character selection

## Changes
- Modified `dashboard/src/lib/components/ChatForm.svelte` 
- Added IME composition check before Enter key handling

Co-authored-by: Ricky Chen <rickychen@Rickys-MacBook-Pro.local>

dashboard/src/lib/components/ChatForm.svelte

@@ -139,6 +139,11 @@
 	}
 
 	function handleKeydown(event: KeyboardEvent) {
+		// Prevent form submission during IME composition (e.g., Chinese, Japanese, Korean input)
+		if (event.isComposing || event.keyCode === 229) {
+			return;
+		}
+		
 		if (event.key === 'Enter' && !event.shiftKey) {
 			event.preventDefault();
 			handleSubmit();