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.

docs/REFACTOR_POLICY.md

@@ -114,7 +114,7 @@ D3RO는 이미 3계층 강제 규칙 체계를 가진다. 본 정책은 이를 *
 |---|---|---|---|
 | W3-1 | 요금 | `packages/core/src/plan-catalog.ts` `PLAN_PRICE_KRW` (Free 0 / Pro 2,900 / Pro+ 8,900, 월). 기존 구독자도 다음 갱신부터 적용 | Deno `_shared/core-contract.generated.ts` (`npm run contract:sync`, CI `contract:check`) |
 | W3-2 | 공개 URL | `packages/core/src/web-urls.ts` — 사이트 `/`, 웹앱 `/app`(Next basePath), `billingUrl()`, `SITE_URLS` | 같은 생성 파일 |
-| W3-3 | 결제 진입 | 웹앱 `/app/billing` 하나. 데스크톱·모바일·사이트·Stripe 복귀는 모두 `billingUrl()`. 복귀 쿼리는 `success=1`/`canceled=1` | — |
+| W3-3 | 결제 진입 | 웹앱 `/app/billing` 하나. 데스크톱·모바일·사이트는 모두 `billingUrl()`. 결제는 페이지 안 Payple 창에서 끝나므로 복귀 쿼리는 없다(Stripe와 `success=1`/`canceled=1` 복귀 쿼리는 2026-09-26 제거) | — |
 | W3-4 | 웹앱 배포 | NAS Docker(`d3ro_voice_web`) → Cloudflare Tunnel 호스트 → 사이트 브리지 워커가 `/app/*`를 전달 | — |
 | W3-5 | 랜딩·다운로드·법률·초대·assetlinks | `site/` 한 벌. apps/web·api-server wwwroot 사본 삭제, 웹앱 `/download`는 사이트로 리다이렉트 | — |
 | W3-6 | 설치 파일 | Forgejo feed만. 저장소에 추적된 바이너리 사본 삭제 | — |