feat: Support customizable welcome text, availability messages, and UI toggles (#11891)

# Pull Request Template

## Description

This PR allows users to dynamically pass custom welcome and availability
messages, along with UI feature toggles, via `window.chatwootSettings`.
If any of the following settings are provided, the widget will use them;
otherwise, it falls back to default behavior.

**New options:**
```
window.chatwootSettings = {
  welcomeTitle: 'Need help?',                        // Custom widget title
  welcomeDescription: 'We’re here to support you.',        // Subtitle in the header
  availableMessage: 'We’re online and ready to chat!', // Shown when team is online
  unavailableMessage: 'We’re currently offline.',      // Shown when team is unavailable

  enableFileUpload: true,          // Enable file attachments
  enableEmojiPicker: true,         // Enable emoji picker in chat input
  enableEndConversation: true     // Allow users to end the conversation
}
```


Fixes
https://linear.app/chatwoot/issue/CW-4589/add-options-to-windowchatwootsettings

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

### Loom video

https://www.loom.com/share/413fc4aa59384366b071450bd19d1bf8?sid=ff30fb4c-267c-4beb-80ab-d6f583aa960d

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] 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/widget/components/ChatAttachment.vue

@@ -24,7 +24,10 @@ export default {
     return { isUploading: false };
   },
   computed: {
-    ...mapGetters({ globalConfig: 'globalConfig/get' }),
+    ...mapGetters({
+      globalConfig: 'globalConfig/get',
+      shouldShowFilePicker: 'appConfig/getShouldShowFilePicker',
+    }),
     fileUploadSizeLimit() {
       return MAXIMUM_FILE_UPLOAD_SIZE;
     },
@@ -40,6 +43,9 @@ export default {
   },
   methods: {
     handleClipboardPaste(e) {
+      // If file picker is not enabled, do not allow paste
+      if (!this.shouldShowFilePicker) return;
+
       const items = (e.clipboardData || e.originalEvent.clipboardData).items;
       // items is a DataTransferItemList object which does not have forEach method
       const itemsArray = Array.from(items);