feat: Add the frontend support for MFA (#12372)

FE support for https://github.com/chatwoot/chatwoot/pull/12290
## Linear:
- https://github.com/chatwoot/chatwoot/issues/486

## Description
This PR implements Multi-Factor Authentication (MFA) support for user
accounts, enhancing security by requiring a second form of verification
during login. The feature adds TOTP (Time-based One-Time Password)
authentication with QR code generation and backup codes for account
recovery.

## Type of change

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

## How Has This Been Tested?

- Added comprehensive RSpec tests for MFA controller functionality
- Tested MFA setup flow with QR code generation
- Verified OTP validation and backup code generation
- Tested login flow with MFA enabled/disabled

## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] 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
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Pranav <pranav@chatwoot.com>
Co-authored-by: iamsivin <iamsivin@gmail.com>
Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
Co-authored-by: Sojan Jose <sojan@pepalo.com>

app/javascript/v3/App.vue

@@ -15,8 +15,10 @@ export default {
     setColorTheme() {
       if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
         this.theme = 'dark';
+        document.documentElement.classList.add('dark');
       } else {
-        this.theme = 'light ';
+        this.theme = 'light';
+        document.documentElement.classList.remove('dark');
       }
     },
     listenToThemeChanges() {
@@ -25,8 +27,10 @@ export default {
       mql.onchange = e => {
         if (e.matches) {
           this.theme = 'dark';
+          document.documentElement.classList.add('dark');
         } else {
           this.theme = 'light';
+          document.documentElement.classList.remove('dark');
         }
       };
     },