fix(collab-web): guard composer submit against IME composition

Both onKeyDown handlers in Composer.tsx committed on Enter without checking
IME composition state, so confirming a Korean/Japanese/Chinese composition
with Enter re-committed the composing character (한글 -> 한글글).

Extract a shouldSubmitOnEnter predicate that returns false while
nativeEvent.isComposing or a composing ref is set, and a useCompositionGuard
hook that tracks compositionstart/compositionend with a deferred reset for
the WebKit case where the confirming Enter keydown fires after
compositionend. Wire both AskEditor and Composer to it.

Fixes #6163

packages/collab-web/test/composer.test.tsx

@@ -1,8 +1,9 @@
 import { describe, expect, it } from "bun:test";
+import type { KeyboardEvent } from "react";
 import { renderToStaticMarkup } from "react-dom/server";
 import type { GuestSnapshot } from "../src/lib/client";
 import { GuestClient } from "../src/lib/client";
-import { Composer } from "../src/components/shell/Composer";
+import { Composer, shouldSubmitOnEnter } from "../src/components/shell/Composer";
 import { encodeBase64Url } from "../src/lib/link";
 
 const LINK = `roomroomroom1234#${encodeBase64Url(new Uint8Array(32))}`;
@@ -77,3 +78,35 @@ describe("Composer host UI requests", () => {
 		expect(submit.disabled).toBe(false);
 	});
 });
+
+type KeyEvt = KeyboardEvent<HTMLTextAreaElement>;
+
+function keydown(key: string, opts: { shiftKey?: boolean; isComposing?: boolean } = {}): KeyEvt {
+	return {
+		key,
+		shiftKey: opts.shiftKey ?? false,
+		nativeEvent: { isComposing: opts.isComposing ?? false },
+	} as KeyEvt;
+}
+
+describe("shouldSubmitOnEnter IME guard", () => {
+	it("submits on a plain Enter with no composition", () => {
+		expect(shouldSubmitOnEnter(keydown("Enter"), false)).toBe(true);
+	});
+
+	it("does not submit while nativeEvent.isComposing is true", () => {
+		expect(shouldSubmitOnEnter(keydown("Enter", { isComposing: true }), false)).toBe(false);
+	});
+
+	it("does not submit while the WebKit composing ref is still set", () => {
+		expect(shouldSubmitOnEnter(keydown("Enter"), true)).toBe(false);
+	});
+
+	it("does not submit on Shift+Enter (newline)", () => {
+		expect(shouldSubmitOnEnter(keydown("Enter", { shiftKey: true }), false)).toBe(false);
+	});
+
+	it("ignores non-Enter keys", () => {
+		expect(shouldSubmitOnEnter(keydown("a"), false)).toBe(false);
+	});
+});