fix: prevent Ctrl+Enter adding extra line break on send (Windows) (#14077)

# Pull Request Template

## Description

This PR includes,
On Windows, pressing **Ctrl+Enter** in the reply editor was inserting an
unintended line break before sending. This led to two issues:

* **Unexpected blank lines**
After adding a line break with Shift+Enter and removing it with
Backspace, the editor looked correct. However, sending with Ctrl+Enter
reintroduced a hidden break, resulting in an extra blank line in the
final message.

* **Selected text being replaced**
When text was selected and Ctrl+Enter was pressed, the selection was
replaced with a line break instead of being sent.

Fixes
https://linear.app/chatwoot/issue/CW-6840/newline-bug-in-the-editor


### **Cause**

Two keyboard handlers responded to **Ctrl+Enter** on Windows:

* ProseMirror (`Mod-Enter`) inserted a hard break
* ReplyBox (`$mod+Enter`) triggered send

The existing guard only checked `metaKey` (Cmd), so it never worked on
Windows. As a result, a line break was inserted just before sending.

### **Solution**

Make the modifier check platform-aware so the editor correctly
intercepts the send shortcut:

* Added `detectOS`, `isMac`, and `OS` constants
* Introduced `hasPressedMod` (uses `metaKey` on macOS, `ctrlKey`
elsewhere)

This ensures Ctrl+Enter sends the message without modifying content,
while keeping existing behavior unchanged.


**NB:** macOS behavior with Cmd+Enter remains unchanged



## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

**Case 1: line break**

1. Type `hello`
2. Press Shift+Enter, then Backspace
3. Press Ctrl+Enter
   → Message contains an unexpected blank new line

**Case 2: Selection replaced**

1. Type two lines using Shift+Enter
2. Select text on the second line
3. Press Ctrl+Enter
   → Selected text is replaced and not sent

### Screencast

**Before**


https://github.com/user-attachments/assets/d6d285a9-260b-4711-8bbd-d0c8519e8d20

**After**



https://github.com/user-attachments/assets/c0ace1f7-5d22-44a2-8e08-22190ee21e61



## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/shared/helpers/KeyboardHelpers.js

@@ -1,3 +1,5 @@
+import { isApple } from './platform';
+
 export const isEnter = e => {
   return e.key === 'Enter';
 };
@@ -14,13 +16,20 @@ export const hasPressedCommand = e => {
   return e.metaKey;
 };
 
+// True when the platform's "command" modifier is held: Cmd (metaKey) on
+// Apple platforms (macOS, iOS/iPadOS hardware keyboards), Ctrl (ctrlKey)
+// elsewhere. Mirrors the `$mod` convention used by tinykeys and
+// prosemirror-keymap so the editor and the app agree on what counts as the
+// send modifier.
+export const hasPressedMod = e => Boolean(isApple() ? e.metaKey : e.ctrlKey);
+
 export const hasPressedEnterAndNotCmdOrShift = e => {
-  return isEnter(e) && !hasPressedCommand(e) && !hasPressedShift(e);
+  return isEnter(e) && !hasPressedMod(e) && !hasPressedShift(e);
 };
 
-export const hasPressedCommandAndEnter = e => {
-  return hasPressedCommand(e) && isEnter(e);
-};
+// Detects the platform-aware "send" shortcut: Cmd+Enter on Apple platforms,
+// Ctrl+Enter on Windows/Linux.
+export const hasPressedCommandAndEnter = e => hasPressedMod(e) && isEnter(e);
 
 // If layout is QWERTZ then we add the Shift+keysToModify to fix an known issue
 // https://github.com/chatwoot/chatwoot/issues/9492