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/mixins/fileUploadMixin.js

@@ -3,6 +3,7 @@ import { useAlert } from 'dashboard/composables';
 import { checkFileSizeLimit } from 'shared/helpers/FileHelper';
 import { getMaxUploadSizeByChannel } from '@chatwoot/utils';
 import { DirectUpload } from 'activestorage';
+import { MAXIMUM_FILE_UPLOAD_SIZE } from 'shared/constants/messages';
 
 export default {
   computed: {
@@ -13,6 +14,11 @@ export default {
 
   methods: {
     maxSizeFor(mime) {
+      // Use default file size limit for private notes
+      if (this.isOnPrivateNote) {
+        return MAXIMUM_FILE_UPLOAD_SIZE;
+      }
+
       return getMaxUploadSizeByChannel({
         channelType: this.inbox?.channel_type,
         medium: this.inbox?.medium, // e.g. 'sms' | 'whatsapp'