feat(billing): embed Stripe Payment Element in onboarding (#21759)

## What & why

Replaces the hosted Stripe Checkout redirect on the onboarding "Choose
your plan" step (credit-card trial) with an inline Stripe **Payment
Element**, so users never leave the app to enter card details.

## How it works

- **Frontend:** a deferred `<Elements mode="setup">` renders the Payment
Element, themed via the Appearance API. On Continue: `elements.submit()`
→ `checkoutSession` mutation creates the trialing subscription
server-side and returns its pending SetupIntent `clientSecret` →
`stripe.confirmSetup()` confirms the card (handling 3DS) → redirect to
the existing `/plan-required/payment-success`.
- **Backend:** new `BILLING_STRIPE_PUBLISHABLE_KEY` config var exposed
via `/client-config`; the card path creates the subscription with
`payment_behavior: default_incomplete` + a free trial (so Stripe
attaches a `pending_setup_intent`) and returns its client secret. The
hosted-Checkout code path is removed.
- The **no-credit-card** trial path is unchanged.
- Billing address collection is **disabled** in the Payment Element to
reduce friction; `automatic_tax` is correspondingly disabled (tax needs
an address — collect it later, e.g. at conversion / via the billing
portal).

## Required before this works
1. Set `BILLING_STRIPE_PUBLISHABLE_KEY` (`pk_…`) on the server (infra
change pending).
2. Run `nx run twenty-front:graphql:generate --configuration=metadata`
against a server exposing the updated schema (see inline note on the
hand-authored document).
3. Verify in Stripe test mode: happy path, 3DS (`4000 0025 0000 3155`),
a decline.

## Verified
typecheck (front + server), oxlint + oxfmt clean,
`client-config.service.spec` passing. Not run here: the app end-to-end /
Stripe test mode and `graphql:generate` (no server/DB in the dev
container).

I've left self-review comments inline flagging cleanup opportunities
plus a couple of architectural/tech-debt items.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01TxCfinXq7abSrbF7aTw2cA

---
_Generated by [Claude
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---------

Co-authored-by: Claude <noreply@anthropic.com>

packages/twenty-server/src/engine/core-modules/client-config/client-config.entity.ts

@@ -165,6 +165,9 @@ export class Billing {
   @Field(() => String, { nullable: true })
   billingUrl?: string;
 
+  @Field(() => String, { nullable: true })
+  stripePublishableKey?: string;
+
   @Field(() => [BillingTrialPeriodDTO])
   trialPeriods: BillingTrialPeriodDTO[];
 }

packages/twenty-server/src/engine/core-modules/client-config/services/client-config.service.spec.ts

@@ -74,6 +74,7 @@ describe('ClientConfigService', () => {
             BILLING_PLAN_REQUIRED_LINK: 'https://billing.example.com',
             BILLING_FREE_TRIAL_WITH_CREDIT_CARD_DURATION_IN_DAYS: 30,
             BILLING_FREE_TRIAL_WITHOUT_CREDIT_CARD_DURATION_IN_DAYS: 7,
+            BILLING_STRIPE_PUBLISHABLE_KEY: 'pk_test_123',
             AUTH_GOOGLE_ENABLED: true,
             AUTH_PASSWORD_ENABLED: true,
             AUTH_MICROSOFT_ENABLED: false,
@@ -121,6 +122,7 @@ describe('ClientConfigService', () => {
         billing: {
           isBillingEnabled: true,
           billingUrl: 'https://billing.example.com',
+          stripePublishableKey: 'pk_test_123',
           trialPeriods: [
             {
               duration: 30,

packages/twenty-server/src/engine/core-modules/client-config/services/client-config.service.ts

@@ -160,6 +160,9 @@ export class ClientConfigService {
       billing: {
         isBillingEnabled: this.twentyConfigService.get('IS_BILLING_ENABLED'),
         billingUrl: this.twentyConfigService.get('BILLING_PLAN_REQUIRED_LINK'),
+        stripePublishableKey: this.twentyConfigService.get(
+          'BILLING_STRIPE_PUBLISHABLE_KEY',
+        ),
         trialPeriods: [
           {
             duration: this.twentyConfigService.get(