fix: add validation for phone number in checkout page

[Prabalpsingh7115]

Overview

1. This PR does the following: -
   a) Added Validation for phone number in checkout page. b) Added a toaster for Internal Server Error & Bad Gateway

Essential Checklist

• [ ] The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• [x] "Allow edits from maintainers" is checked. (See here for instructions on how to enable it.)
• [x] The PR is made from a branch that's not called "main/master".

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