fix(saas): show team invitation banner in SaaS web build (#6612)

## Problem

When a user is invited to a team, the SaaS web app shows **no invitation
banner** — even though the pending invite is returned by
`/api/v1/team/invitations/pending` on refresh.

## Root causes

1. **Never rendered in SaaS.** `TeamInvitationBanner` only existed in
`desktop/`, wired solely into `DesktopBannerInitializer`. The SaaS
banner stack rendered only `<UpgradeBanner />`.
2. **Single banner slot.** `BannerContext` holds one node; `setBanner`
replaces it. `TrialStatusBanner` called `setBanner(null)` when there was
no active trial (and re-fired once `trialStatus` resolved async), wiping
any other banner.
3. **Shadowing was too fragile.** A first attempt shadowed the
proprietary `UpgradeBannerInitializer` from the saas layer, but
`vite-tsconfig-paths` resolves the `@app` specifier once at dev-server
start — a newly-added shadow of an already-resolved module isn't picked
up on a browser refresh, only a full restart. So the proprietary
initializer kept running and no invite banner appeared (while the SaaS
team context still fetched + populated the invite, which is why the
pending call was visible).

## Fix

- Add `saas/components/shared/TeamInvitationBanner.tsx` — ported from
desktop, minus the desktop `connectionMode` gate and explicit billing
refresh (SaaS `acceptInvitation` already refreshes credits + session).
- Render it **inline in `saas/routes/Landing.tsx`** next to
`GuestUserBanner` — a new import specifier in an existing file
(HMR-friendly), unambiguously inside `SaaSTeamProvider`, mirroring the
proven `GuestUserBanner` pattern. No dependency on the single banner
slot.
- **Remove `TrialStatusBanner`** (trials are being retired) so it can't
clobber banners. Also drops the stale mention from the stripe-lazy-load
test comment.

## Verification

- `tsc --noEmit -p tsconfig.saas.vite.json`: clean in touched files;
total unchanged from baseline (37 pre-existing, unrelated).
- Manual: pull + verify the Accept/Decline banner appears for an account
with a pending invite.

frontend/editor/src/core/tests/stubbed/stripe-lazy-load.spec.ts

@@ -4,9 +4,9 @@ import { test, expect } from "@app/tests/helpers/stub-test-base";
  * Verifies that the Stripe SDK (@stripe/stripe-js, @stripe/react-stripe-js,
  * and the js.stripe.com remote script) is NOT fetched on cold page loads —
  * only when the checkout modal actually mounts. The proprietary
- * CheckoutProvider, the SaaS TrialStatusBanner, and the SaaS Plan settings
- * page all gate the modal behind React.lazy + a conditional render so the
- * Stripe chunk lives in its own async bundle.
+ * CheckoutProvider and the SaaS Plan settings page gate the modal behind
+ * React.lazy + a conditional render so the Stripe chunk lives in its own
+ * async bundle.
  */
 
 const STRIPE_URL_FRAGMENTS = [