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.

apps/desktop/tests/e2e/launch_readiness.spec.ts

@@ -120,22 +120,11 @@ test.describe('Launch Readiness E2E & Visual Verification', () => {
     await window.waitForTimeout(500);
   });
 
-  test('03. Should open the server-authoritative Stripe CheckoutModal', async () => {
-    // Trigger checkout via "Remove ads with Pro" link in AdBanner
+  test('03. Should expose the web billing upgrade link without an in-app checkout modal', async () => {
+    // "Remove ads with Pro" opens billingUrl({ tier }) in the external browser (LICENSE.OPEN_BILLING).
     const removeAdsLink = window.locator('text=/Remove ads with Pro/').first();
     if (await removeAdsLink.isVisible()) {
-      await removeAdsLink.click();
-      await window.waitForTimeout(600);
-
-      // Verify Checkout Modal is visible
-      await expect(window.locator('text=/D3RO Voice Secure Checkout/').first()).toBeVisible();
-
-      // Capture screenshot of CheckoutModal
-      await window.screenshot({ path: path.join(SCREENSHOT_DIR, '04_checkout_modal_stripe.png') });
-
-      // Close modal
-      await window.keyboard.press('Escape');
-      await window.waitForTimeout(400);
+      await expect(window.locator('text=/D3RO Voice Secure Checkout/')).toHaveCount(0);
     }
   });
 });