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.

packages/core/src/ipc-channels.ts

@@ -527,14 +527,6 @@ export const IPC_CHANNELS = {
     CHECK_REFUND: 'support:checkRefund',
   },
 
-  // ── Multi-PG Payment & Billing ──
-  PAYMENT: {
-    CREATE_CHECKOUT_SESSION: 'payment:createCheckoutSession',
-    VERIFY_PAYMENT: 'payment:verifyPayment',
-    GET_SUBSCRIPTION_STATUS: 'payment:getSubscriptionStatus',
-    CANCEL_SUBSCRIPTION: 'payment:cancelSubscription',
-  },
-
   // ── Input telemetry (수집·동의·리포트) ──
   INPUT_TELEMETRY: {
     GET_STATE: 'inputTelemetry:getState',