fix(06-05): make AuthSplash dead-end state reachable + persist redirect guard

- CalendarShell now captures maybeRedirectToLogin() return value in meQuery.isError effect
- When the one-shot guard is exhausted (returns false), arm loginRedirectExhausted state
- Render AuthSplash state=dead-end (tap-to-retry) when guard is exhausted, not indefinite redirecting spinner
- Reset loginRedirectExhausted on successful auth (meQuery.isSuccess) for session recovery
- Add sessionStorage.clear() to beforeEach so CalendarShell tests are isolated
- RED test committed in prior commit (36ef7a0)

apps/pwa/src/components/CalendarShell.test.tsx

@@ -140,6 +140,8 @@ function renderWithClient(ui: React.ReactElement) {
 describe('CalendarShell — CAL-03 render smoke', () => {
   beforeEach(() => {
     vi.clearAllMocks()
+    // Reset the one-shot redirect guard between tests
+    sessionStorage.clear()
 
     // Default mock responses
     ;(fetchMe as Mock).mockResolvedValue({