feat(voice): move WhatsApp calling toggle to its own Calls tab

Mirror the Twilio voice configuration flow: split the WhatsApp Cloud
calling-enabled toggle out of the generic Configuration tab into a
dedicated Calls tab so the surface for telephony settings is consistent
across providers.

- Added WhatsappCallingPage.vue with the toggle, business phone display,
  and a how-it-works blurb (no extra credentials needed since the
  embedded-signup token already grants the call scopes).
- Settings.vue: register the page and surface a Calls tab on
  WhatsApp Cloud + embedded-signup inboxes.
- ConfigurationPage.vue: drop the inline calling-enabled section, the
  watcher that auto-saved on every toggle flip, and the now-orphan
  updateWhatsAppCallingEnabled method.
- en/inboxMgmt.json: TABS.CALLS label + WHATSAPP_CALLING.* strings.

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

@@ -637,7 +637,8 @@
       "BOT_CONFIGURATION": "Bot Configuration",
       "ACCOUNT_HEALTH": "Account Health",
       "CSAT": "CSAT",
-      "VOICE": "Voice"
+      "VOICE": "Voice",
+      "CALLS": "Calls"
     },
     "VOICE_CONFIGURATION": {
       "ENABLE_VOICE": {
@@ -648,6 +649,20 @@
         "DESCRIPTION": "Voice calling requires Twilio API Key credentials. These are used to generate tokens for agent voice connections."
       }
     },
+    "WHATSAPP_CALLING": {
+      "ENABLE": {
+        "LABEL": "Enable WhatsApp Calling",
+        "DESCRIPTION": "Allow agents to receive and place WhatsApp Cloud calls on this inbox. Customers can call this business number directly from WhatsApp."
+      },
+      "PHONE_NUMBER": {
+        "LABEL": "Business phone number",
+        "HELP_TEXT": "WhatsApp number that customers will call."
+      },
+      "HOW_IT_WORKS": {
+        "LABEL": "How it works",
+        "DESCRIPTION": "Calls are placed peer-to-peer between the agent's browser and Meta — no extra credentials are required. Make sure the agent's browser has microphone permission for this site."
+      }
+    },
     "CHANNEL_PREFERENCES": "Channel Preferences",
     "WIDGET_FEATURES": "Widget features",
     "ACCOUNT_HEALTH": {