chore: Improve portal settings validation and error handling (#12134)

# Pull Request Template

## Description

This PR includes:

1. Previously, the URL fields accepted any value starting with `https`.
Added proper URL validation to ensure valid URLs are entered.
2. Fixed an issue where form save errors displayed `[object Object]` in
the toast due to inconsistent error formatting from the backend.


Fixes https://linear.app/chatwoot/issue/CW-5389/help-center-bugs

## 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/63ddca2c5e2e45c99b66153ed146524f?sid=fd25331b-6b67-4722-9b36-1213496a0741

## 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

app/javascript/dashboard/i18n/locale/en/helpCenter.json

@@ -732,7 +732,7 @@
         "HOME_PAGE_LINK": {
           "LABEL": "Home page link",
           "PLACEHOLDER": "Portal home page link",
-          "ERROR": "Invalid URL. The Home page link must start with 'http://' or 'https://'."
+          "ERROR": "Enter a valid URL. The Home page link must start with 'http://' or 'https://'."
         },
         "SLUG": {
           "LABEL": "Slug",