refactor(billing): remove Stripe; payments are Payple (web) and Google Play (mobile)

Stripe is not used. Keeping its checkout, portal and webhook paths meant a
second payment provider, a second return-URL format and dead UI.

- Delete the stripe-checkout, stripe-portal and stripe-webhook functions and
  their config; billing-catalog serves Payple prices only, and the web parser
  rejects a catalog that still mixes in Stripe prices.
- Web: drop the Stripe checkout/portal buttons, provider toggle and return
  notices; billing shows Payple only. Past rows with provider='stripe' are
  still displayed ("Stripe (종료)") with a support contact instead of a portal.
- Desktop: delete the Stripe checkout modal, payment IPC channels, preload
  namespace and their types; "Remove ads with Pro" opens the web billing page
  via license.openBilling. Support/refund copy names Payple.
- billingUrl() loses the Stripe-only success/canceled result option; the
  Deno contract is regenerated.
- Migrations and the DB's accepted provider values are untouched (history).
- Docs and the backlog record the removal (MON-04, EXT-STRIPE-01, GAP-BILL-03).

Verified: typecheck (desktop/web/admin/api-client/mobile), contract:check,
deno check all functions, deno test 80/80, desktop 1478/1480 on the Electron
runtime (2 known environment failures), web and admin builds, release
metadata and mobile boundary self-tests, eslint on changed files.

server/supabase/functions/_shared/core-contract.generated.ts

@@ -18,7 +18,7 @@ export type PlanTier = 'free' | PaidPlanTier
 
 /**
  * 월 요금(원). Payple 청구 금액이며, 기존 구독자도 다음 갱신부터 이 값으로 청구된다.
- * Stripe(USD)·Google Play 가격은 각 콘솔에서 따로 관리한다.
+ * Google Play 가격은 Play Console에서 따로 관리한다. 웹 결제는 Payple(KRW) 하나다.
  */
 export const PLAN_PRICE_KRW: Readonly<Record<PlanTier, number>> = {
   free: 0,
@@ -122,19 +122,13 @@ export const SITE_URLS = {
   acceptInvite: `${PUBLIC_SITE_ORIGIN}/accept-invite/`,
 } as const
 
-/** 결제 결과로 돌아올 때 붙는 쿼리. apps/web 결제 페이지가 읽는 이름과 같다. */
-export type BillingReturn = 'success' | 'canceled'
-
 /**
- * 웹 결제 페이지 URL.
+ * 웹 결제 페이지 URL. 결제는 이 페이지 안의 Payple 창에서 끝나므로 복귀 쿼리가 없다.
  * - `tier`: 고를 요금제를 미리 선택한다.
- * - `result`: 외부 결제(Stripe 등)에서 돌아올 때의 결과.
  */
-export function billingUrl(options: { tier?: PaidPlanTier; result?: BillingReturn } = {}): string {
+export function billingUrl(options: { tier?: PaidPlanTier } = {}): string {
   const params = new URLSearchParams()
   if (options.tier) params.set('tier', options.tier)
-  if (options.result === 'success') params.set('success', '1')
-  if (options.result === 'canceled') params.set('canceled', '1')
   const query = params.toString()
   return `${WEB_APP_URL}/billing${query ? `?${query}` : ''}`
 }