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/sdk/IFrameHelper.js

@@ -166,6 +166,13 @@ export const IFrameHelper = {
         darkMode: window.$chatwoot.darkMode,
         showUnreadMessagesDialog: window.$chatwoot.showUnreadMessagesDialog,
         campaignsSnoozedTill,
+        welcomeTitle: window.$chatwoot.welcomeTitle,
+        welcomeDescription: window.$chatwoot.welcomeDescription,
+        availableMessage: window.$chatwoot.availableMessage,
+        unavailableMessage: window.$chatwoot.unavailableMessage,
+        enableFileUpload: window.$chatwoot.enableFileUpload,
+        enableEmojiPicker: window.$chatwoot.enableEmojiPicker,
+        enableEndConversation: window.$chatwoot.enableEndConversation,
       });
       IFrameHelper.onLoad({
         widgetColor: message.config.channelConfig.widgetColor,