feat(onboarding): honor return_to hint in TikTok OAuth callback (#14569)

When connecting a TikTok inbox during onboarding, the OAuth flow used to
drop users in inbox settings, breaking onboarding. The OAuth start
endpoint now accepts an optional `return_to=onboarding` hint, carried
tamper-proof inside the signed `state` (a claim on TikTok's signed JWT),
and the callback uses it to return the user to the onboarding
inbox-setup screen. Without the hint, behavior is unchanged.

This is the backend half only; the frontend that sends
`return_to=onboarding` ships separately.

## What changed
- `Tiktok::IntegrationHelper`: the signed JWT carries an optional
`return_to` claim, added only when present (a request without it is
byte-identical to before); added `tiktok_token_return_to` to read it;
`decode_token` now returns the full payload and `verify_tiktok_token`
derives the account id from it.
- `Tiktok::AuthorizationsController#create` passes `params[:return_to]`
into the token.
- `Tiktok::CallbacksController` redirects to the onboarding inbox-setup
screen when `return_to == 'onboarding'`, before the normal
settings/agents redirect.
- Added the `app_onboarding_inbox_setup` route (shared with the sibling
Gmail/Outlook and Instagram PRs — keep a single copy on merge to avoid a
duplicate route name).

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

app/controllers/api/v1/accounts/tiktok/authorizations_controller.rb

@@ -3,7 +3,7 @@ class Api::V1::Accounts::Tiktok::AuthorizationsController < Api::V1::Accounts::O
 
   def create
     redirect_url = Tiktok::AuthClient.authorize_url(
-      state: generate_tiktok_token(Current.account.id)
+      state: generate_tiktok_token(Current.account.id, params[:return_to])
     )
 
     if redirect_url