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/TeamAvailability.vue

@@ -29,6 +29,8 @@ export default {
   computed: {
     ...mapGetters({
       widgetColor: 'appConfig/getWidgetColor',
+      availableMessage: 'appConfig/getAvailableMessage',
+      unavailableMessage: 'appConfig/getUnavailableMessage',
     }),
     textColor() {
       return getContrastingTextColor(this.widgetColor);
@@ -40,6 +42,11 @@ export default {
         id: agent.id,
       }));
     },
+    headerMessage() {
+      return this.isOnline
+        ? this.availableMessage || this.$t('TEAM_AVAILABILITY.ONLINE')
+        : this.unavailableMessage || this.$t('TEAM_AVAILABILITY.OFFLINE');
+    },
     isOnline() {
       const { workingHoursEnabled } = this.channelConfig;
       const anyAgentOnline = this.availableAgents.length > 0;
@@ -71,12 +78,8 @@ export default {
   >
     <div class="flex items-center justify-between gap-2">
       <div class="flex flex-col gap-1">
-        <div class="font-medium text-n-slate-12">
-          {{
-            isOnline
-              ? $t('TEAM_AVAILABILITY.ONLINE')
-              : $t('TEAM_AVAILABILITY.OFFLINE')
-          }}
+        <div class="font-medium text-n-slate-12 line-clamp-2">
+          {{ headerMessage }}
         </div>
         <div class="text-n-slate-11">
           {{ replyWaitMessage }}