Tune server-fn rate limits and handle 429 in the UI

[peetzweg]

Follow-up to #121, which added a per-IP rate limit on the server-function endpoints (burst 40, sustained 20/min — constants in src/start.ts).

Why: the numbers are an educated guess (~5 RPC calls per client-side navigation), not measured. And when a legit heavy user does trip the limit, the frontend has no handling for a 429 — TanStack Query will surface it as a generic error state, which reads as "the site is broken" instead of "slow down a moment".

What to do:

• After some real traffic, check how close normal browsing gets to the limit and adjust burst/sustained accordingly
• Handle 429 responses in the query/error layer: show a friendly "too many requests — retrying shortly" state and respect the Retry-After header instead of hammering retries