feat: ensure signup verification [UPM-14] (#13858)

Previously, signing up gave immediate access to the app. Now,
unconfirmed users are redirected to a verification page where they can
resend the confirmation email.

- After signup, the user is routed to `/auth/verify-email` instead of
the dashboard
- After login, unconfirmed users are redirected to the verification page
- The dashboard route guard catches unconfirmed users and redirects them
- `active_for_authentication?` is removed from the sessions controller
so unconfirmed users can authenticate — the frontend gates access
instead
- If the user visits the verification page after already confirming,
they're automatically redirected to the dashboard
- No session is issued until the user is verified

<details><summary>Demo</summary>
<p>

#### Fresh Signup


https://github.com/user-attachments/assets/abb735e5-7c8e-44a2-801c-96d9e4823e51

#### Google Fresh Signup


https://github.com/user-attachments/assets/ab9e389a-a604-4a9d-b492-219e6d94ee3f


#### Create new account from Dashboard


https://github.com/user-attachments/assets/c456690d-1946-4e0b-834b-ad8efcea8369



</p>
</details>

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/controllers/auth/resend_confirmations_controller.rb

@@ -0,0 +1,18 @@
+# Unauthenticated endpoint for resending confirmation emails during signup.
+# This is a standalone controller (not on DeviseOverrides::ConfirmationsController)
+# because OmniAuth middleware intercepts all POST /auth/* routes as provider
+# callbacks, and Devise controller filters cause 307 redirects for custom actions.
+# Inherits from ActionController::API to avoid both issues entirely.
+# Rate-limited by Rack::Attack (IP + email) and gated by hCaptcha.
+class Auth::ResendConfirmationsController < ActionController::API
+  def create
+    return head(:ok) unless ChatwootCaptcha.new(params[:h_captcha_client_response]).valid?
+
+    email = params[:email]
+    return head(:ok) unless email.is_a?(String)
+
+    user = User.from_email(email.strip.downcase)
+    user&.send_confirmation_instructions unless user&.confirmed?
+    head :ok
+  end
+end