[fix][CLNP-7811] Treat whitespace-only input as empty for typing state

The previous empty-check used `getTextContentWithoutZeroWidthSpace(...) ===
''` (no trim), while the same handler's `setIsInput` call used
`hasTextContentWithoutZeroWidthSpace` (trim-aware). This split meant a user
who typed text and then deleted it down to whitespace/newlines saw the send
button disappear (functionally empty UI) but the typing indicator stayed
visible to peers until the SDK server-side timeout expired.

Use `hasTextContentWithoutZeroWidthSpace` for both checks so typing state
follows the same trim semantics as the send-button state. The DOM read is
also collapsed into a single call.

Add 2 tests:
- onStopTyping fires when input becomes whitespace-only after typing
- onStartTyping does not fire on whitespace-only input

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sf-tyler-jeong

src/ui/MessageInput/__tests__/MessageInput.spec.js

@@ -318,6 +318,33 @@ describe('ui/MessageInput', () => {
 
       expect(onStopTyping).toHaveBeenCalledTimes(1);
     });
+
+    it('should call onStopTyping when input becomes whitespace-only after typing', () => {
+      const onStartTyping = jest.fn();
+      const onStopTyping = jest.fn();
+      render(<MessageInput onSendMessage={noop} onStartTyping={onStartTyping} onStopTyping={onStopTyping} />);
+
+      const input = screen.getByRole('textbox');
+      input.textContent = 'hello';
+      fireEvent.input(input);
+      input.textContent = '   ';
+      fireEvent.input(input);
+
+      expect(onStopTyping).toHaveBeenCalledTimes(1);
+    });
+
+    it('should not call onStartTyping when input contains only whitespace', () => {
+      const onStartTyping = jest.fn();
+      const onStopTyping = jest.fn();
+      render(<MessageInput onSendMessage={noop} onStartTyping={onStartTyping} onStopTyping={onStopTyping} />);
+
+      const input = screen.getByRole('textbox');
+      input.textContent = '   ';
+      fireEvent.input(input);
+
+      expect(onStartTyping).not.toHaveBeenCalled();
+      expect(onStopTyping).not.toHaveBeenCalled();
+    });
   });
 });
 

src/ui/MessageInput/index.tsx

@@ -526,17 +526,15 @@ const MessageInput = React.forwardRef<HTMLInputElement, MessageInputProps>((prop
             useMentionInputDetection();
           }}
           onInput={() => {
-            const isEmpty = getTextContentWithoutZeroWidthSpace(internalRef?.current) === '';
-            if (isEmpty) {
-              if (wasTypingRef.current) {
-                onStopTyping();
-                wasTypingRef.current = false;
-              }
-            } else {
+            const hasContent = hasTextContentWithoutZeroWidthSpace(internalRef?.current);
+            if (hasContent) {
               onStartTyping();
               wasTypingRef.current = true;
+            } else if (wasTypingRef.current) {
+              onStopTyping();
+              wasTypingRef.current = false;
             }
-            setIsInput(hasTextContentWithoutZeroWidthSpace(internalRef?.current));
+            setIsInput(hasContent);
             useMentionedLabelDetection();
           }}
           onPaste={(e) => {