Show 'creator not signed in' message instead of infinite spinner

[zhiganov]

Problem

The poll page shows an infinite loading spinner in two scenarios, with no feedback to the user.

Scenario 1: Railway cold start

When the server has been idle, Railway puts it to sleep. The first request takes several seconds to wake it up. The client shows a spinner but has no timeout, retry, or "still loading" message — it just spins indefinitely until the server responds.

Scenario 2: Creator session expired

When the poll creator's ATProto session has expired, endpoints that require the creator's OAuth session (e.g. response submission) may hang or fail silently.

Note: poll VIEWING uses unauthenticated PDS reads and should NOT be affected by session expiry. If viewing is hanging, it's likely Scenario 1 (cold start) rather than a session issue.

Current behavior

Endless preloader animation on white background. User has no idea what's wrong or whether to wait.

Expected behavior

• Show a "still loading, please wait..." message after 3-5 seconds
• After 10-15 seconds, show a retry button or an error message
• If response submission fails due to expired creator session, show a clear message: "The poll creator needs to re-authorize — responses are temporarily unavailable"
• Never spin indefinitely with no feedback

Related

• Decouple response submission from creator's active OAuth session #42 — Decouple response submission from creator's session (root cause for Scenario 2)