Fix infinite redirect loop after deleting an organization

[Cryptoteep]

Problem

Reported in l3montree-dev/devguard#2131: deleting an organization from the
settings page ("Danger Zone") redirects the user back to /, but they get
stuck bouncing back into the page of the organization that was just
deleted (which no longer exists). A manual page refresh fixes it.

Root cause

handleDeleteOrganization in
src/app/(loading-group)/[organizationSlug]/settings/page.tsx called
router.push("/") after a successful deletion. That is a client-side
transition, so the root layout's SessionShell (a Server Component that
fetches the session and organization list once) is not re-executed —
the organization list kept in SessionContext still contained the
just-deleted organization.

On /, src/app/page.tsx reads localStorage.lastActiveOrg (which still
pointed at the deleted org, since it is set whenever an org page loads
successfully — including the settings page the user just deleted the org
from) and checks whether it exists in the (stale) organization list. Since
the stale list still contained the deleted org, the app redirected right
back into /​<deleted-org-slug>, which 404s server-side and leaves the
user stuck.

Fix

• Clear lastActiveOrg from localStorage when it points at the
  organization that was just deleted (new helper
  clearStaleLastActiveOrg in src/services/organizationService.ts, unit
  tested).
• Navigate to / with a full page load (window.location.href = "/",
  a pattern already used elsewhere in this codebase) instead of
  router.push, so the session/organization list is refetched from the
  server and no longer contains the deleted organization.

This is a minimal, targeted fix — no unrelated files were touched.

Testing

• Added src/services/organizationService.test.ts covering
  clearStaleLastActiveOrg (removes stale entry, leaves unrelated entry
  untouched, no-ops when unset).
• npx jest — all pre-existing tests pass except one unrelated,
  pre-existing failure in useAutosetup.test.tsx (fails identically on
  main before this change — a sonner/toast mocking issue unrelated to
  this fix).
• npx tsc --noEmit — no errors.

Fixes l3montree-dev/devguard#2131