chore: Default file limits for private notes and reset attachment on mode switch (#12310)

# Pull Request Template

## Description

This PR fixes the handling of attachments and file limits in reply
modes:

* Applies **default file size and type limits** for private notes
* **Resets attachments** automatically when switching reply modes


## Type of change

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

## How Has This Been Tested?

### Loom video

https://www.loom.com/share/abad3e6a0383405ea5f31314c1494f2f?sid=38715fd0-e305-4a9b-8f4d-fc6a6e5c0833


## 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
- [ ] 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

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/javascript/dashboard/composables/useFileUpload.js

@@ -4,14 +4,16 @@ import { useI18n } from 'vue-i18n';
 import { DirectUpload } from 'activestorage';
 import { checkFileSizeLimit } from 'shared/helpers/FileHelper';
 import { getMaxUploadSizeByChannel } from '@chatwoot/utils';
+import { MAXIMUM_FILE_UPLOAD_SIZE } from 'shared/constants/messages';
 
 /**
  * Composable for handling file uploads in conversations
  * @param {Object} options
  * @param {Object} options.inbox - Current inbox object (has channel_type, medium, etc.)
  * @param {Function} options.attachFile - Callback to handle file attachment
+ * @param {boolean} options.isPrivateNote - Whether the upload is for a private note
  */
-export const useFileUpload = ({ inbox, attachFile }) => {
+export const useFileUpload = ({ inbox, attachFile, isPrivateNote = false }) => {
   const { t } = useI18n();
 
   const accountId = useMapGetter('getCurrentAccountId');
@@ -20,12 +22,18 @@ export const useFileUpload = ({ inbox, attachFile }) => {
   const globalConfig = useMapGetter('globalConfig/get');
 
   // helper: compute max upload size for a given file's mime
-  const maxSizeFor = mime =>
-    getMaxUploadSizeByChannel({
+  const maxSizeFor = mime => {
+    // Use default file size limit for private notes
+    if (isPrivateNote) {
+      return MAXIMUM_FILE_UPLOAD_SIZE;
+    }
+
+    return getMaxUploadSizeByChannel({
       channelType: inbox?.channel_type,
       medium: inbox?.medium, // e.g. 'sms' | 'whatsapp' | etc.
       mime, // e.g. 'image/png'
     });
+  };
 
   const alertOverLimit = maxSizeMB =>
     useAlert(