feat: Bulk actions for contacts (#12763)

Introduces APIs and UI for bulk actions in contacts table. The initial
action available will be assign labels

Fixes: #8536 #12253 

## Screens

<img width="1350" height="747" alt="Screenshot 2025-10-29 at 4 05 08 PM"
src="https://github.com/user-attachments/assets/0792dff5-0371-4b2e-bdfb-cd32db773402"
/>
<img width="1345" height="717" alt="Screenshot 2025-10-29 at 4 05 19 PM"
src="https://github.com/user-attachments/assets/ae510404-c6de-4c15-a720-f6d10cdac25b"
/>

---------

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

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

@@ -24,7 +24,7 @@
     },
     "LABELS": {
       "ASSIGN_LABELS": "Assign labels",
-      "NO_LABELS_FOUND": "No labels found for",
+      "NO_LABELS_FOUND": "No labels found",
       "ASSIGN_SELECTED_LABELS": "Assign selected labels",
       "ASSIGN_SUCCESFUL": "Labels assigned successfully.",
       "ASSIGN_FAILED": "Failed to assign labels. Please try again."

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

@@ -572,6 +572,16 @@
       "ACTIVE_EMPTY_STATE_TITLE": "No contacts are active at the moment 🌙"
     }
   },
+  "CONTACTS_BULK_ACTIONS": {
+    "ASSIGN_LABELS": "Assign Labels",
+    "ASSIGN_LABELS_SUCCESS": "Labels assigned successfully.",
+    "ASSIGN_LABELS_FAILED": "Failed to assign labels",
+    "DESCRIPTION": "Select the labels you want to add to the selected contacts.",
+    "NO_LABELS_FOUND": "No labels available yet.",
+    "SELECTED_COUNT": "{count} selected",
+    "CLEAR_SELECTION": "Clear selection",
+    "SELECT_ALL": "Select all ({count})"
+  },
 
   "COMPOSE_NEW_CONVERSATION": {
     "CONTACT_SEARCH": {