perf: update the logic to purchase credits (#12998)

## Description

- Replaces Stripe Checkout session flow with direct card charging for AI
credit top-ups
- Adds a two-step confirmation modal (select package → confirm purchase)
for better UX
- Creates Stripe invoice directly and charges the customer's default
payment method immediately

## Type of change

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

## How Has This Been Tested?

- Using the specs
- UI manual test cases

<img width="945" height="580" alt="image"
src="https://github.com/user-attachments/assets/52bdad46-cd0e-4927-b13f-54c6b6353bcc"
/>

<img width="945" height="580" alt="image"
src="https://github.com/user-attachments/assets/231bc7e9-41ac-440d-a93d-cba45a4d3e3e"
/>


## 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: Shivam Mishra <scm.mymail@gmail.com>

config/locales/en.yml

@@ -127,6 +127,8 @@ en:
       invalid_credits: Invalid credits amount
       invalid_option: Invalid topup option
       plan_not_eligible: Top-ups are only available for paid plans. Please upgrade your plan first.
+      stripe_customer_not_configured: Stripe customer not configured
+      no_payment_method: No payment methods found. Please add a payment method before making a purchase.
   profile:
     mfa:
       enabled: MFA enabled successfully

config/routes.rb

@@ -24,7 +24,6 @@ Rails.application.routes.draw do
     get '/app/accounts/:account_id/settings/inboxes/new/:inbox_id/agents', to: 'dashboard#index', as: 'app_instagram_inbox_agents'
     get '/app/accounts/:account_id/settings/inboxes/:inbox_id', to: 'dashboard#index', as: 'app_instagram_inbox_settings'
     get '/app/accounts/:account_id/settings/inboxes/:inbox_id', to: 'dashboard#index', as: 'app_email_inbox_settings'
-    get '/app/accounts/:account_id/settings/billing', to: 'dashboard#index', as: 'app_account_billing_settings'
 
     resource :widget, only: [:show]
     namespace :survey do