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

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---------

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

packages/twenty-client-sdk/src/metadata/generated/schema.graphql

@@ -1610,6 +1610,11 @@ type BillingPlan {
   meteredProducts: [BillingMeteredProduct!]!
 }
 
+type BillingPaymentIntent {
+  clientSecret: String!
+  paymentIntentType: String!
+}
+
 type BillingSession {
   url: String
 }
@@ -1879,6 +1884,7 @@ enum ModelFamily {
 type Billing {
   isBillingEnabled: Boolean!
   billingUrl: String
+  stripePublishableKey: String
   trialPeriods: [BillingTrialPeriod!]!
 }
 
@@ -3286,6 +3292,7 @@ type Mutation {
   skipBookOnboardingStep: OnboardingStepSuccess!
   updateOneApplicationVariable(key: String!, value: String!, applicationId: UUID!): Boolean!
   checkoutSession(recurringInterval: SubscriptionInterval!, plan: BillingPlanKey! = PRO, requirePaymentMethod: Boolean! = true, successUrlPath: String): BillingSession!
+  createSubscriptionPaymentIntent(recurringInterval: SubscriptionInterval!, plan: BillingPlanKey! = PRO, requirePaymentMethod: Boolean! = true, successUrlPath: String, idempotencyKey: String!): BillingPaymentIntent!
   switchSubscriptionInterval: BillingUpdate!
   switchBillingPlan: BillingUpdate!
   cancelSwitchBillingPlan: BillingUpdate!