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/services/contacts/bulk_action_service.rb

@@ -0,0 +1,32 @@
+class Contacts::BulkActionService
+  def initialize(account:, user:, params:)
+    @account = account
+    @user = user
+    @params = params.deep_symbolize_keys
+  end
+
+  def perform
+    return assign_labels if labels_to_add.any?
+
+    Rails.logger.warn("Unknown contact bulk operation payload: #{@params.keys}")
+    { success: false, error: 'unknown_operation' }
+  end
+
+  private
+
+  def assign_labels
+    Contacts::BulkAssignLabelsService.new(
+      account: @account,
+      contact_ids: ids,
+      labels: labels_to_add
+    ).perform
+  end
+
+  def ids
+    Array(@params[:ids]).compact
+  end
+
+  def labels_to_add
+    @labels_to_add ||= Array(@params.dig(:labels, :add)).reject(&:blank?)
+  end
+end

app/services/contacts/bulk_assign_labels_service.rb

@@ -0,0 +1,19 @@
+class Contacts::BulkAssignLabelsService
+  def initialize(account:, contact_ids:, labels:)
+    @account = account
+    @contact_ids = Array(contact_ids)
+    @labels = Array(labels).compact_blank
+  end
+
+  def perform
+    return { success: true, updated_contact_ids: [] } if @contact_ids.blank? || @labels.blank?
+
+    contacts = @account.contacts.where(id: @contact_ids)
+
+    contacts.find_each do |contact|
+      contact.add_labels(@labels)
+    end
+
+    { success: true, updated_contact_ids: contacts.pluck(:id) }
+  end
+end