chore: Enable flexible whatsapp onboarding (Manual + Embedded Signup) options (#12344)

We recently introduced the WhatsApp Embedded Signup flow in Chatwoot to
simplify onboarding. However, we discovered two important limitations:
Some customers’ numbers are already linked to an Embedded Signup, which
blocks re-use. Tech providers cannot onboard their own numbers via
Embedded Signup.
As a result, we need to support both Manual and Embedded Signup flows to
cover all scenarios.

### Problem

- Current UI only offers the Embedded Signup option.
- Customers who need to reuse existing numbers (already connected to
WABA) or tech providers testing their own numbers get stuck.
- Manual flow exists but is no longer exposed in the UX

**Current Embedded Signup screen**
<img width="2564" height="1250" alt="CleanShot 2025-08-21 at 21 58
07@2x"
src="https://github.com/user-attachments/assets/c3de4cf1-cae6-4a0e-aa9c-5fa4e2249c0e"
/>

**Current Manual Setup screen**
<img width="2568" height="1422" alt="CleanShot 2025-08-21 at 22 00
25@2x"
src="https://github.com/user-attachments/assets/96408f97-3ffe-42d1-9019-a511e808f5ac"
/>


###  Solution

- Design a dual-path UX in the Create WhatsApp Inbox step that:
- Offers Embedded Signup (default/recommended) for new numbers and
businesses.
- Offers Manual Setup for advanced users, existing linked numbers, and
tech providers.

<img width="2030" height="1376" alt="CleanShot 2025-09-01 at 14 13
16@2x"
src="https://github.com/user-attachments/assets/6f17e5a2-a2fd-40fb-826a-c9ee778be795"
/>

---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: iamsivin <iamsivin@gmail.com>

config/features.yml

@@ -184,6 +184,7 @@
 - name: whatsapp_embedded_signup
   display_name: WhatsApp Embedded Signup
   enabled: false
+  deprecated: true
 - name: whatsapp_campaign
   display_name: WhatsApp Campaign
   enabled: false