refactor(constants): zentrale SW-Timing-Konstanten + minor cleanups

- src/lib/constants.ts: SW_VERSION_QUERY_TIMEOUT_MS, SW_UPDATE_POLL_INTERVAL_MS
- pwa.svelte.ts: nutzt die Konstanten statt 1500/30*60_000
- cache-strategy.ts / diff-manifest.ts: RequestShape/ManifestDiff entkapselt
  (intern statt export, da nirgends extern importiert)
- recipes/[id]/image: deutsche Fehlermeldungen auf Englisch (Konsistenz
  mit allen anderen Endpoints)

Findings aus REVIEW-2026-04-18.md (Quick-Wins 6+7) und dead-code.md

src/lib/constants.ts

@@ -0,0 +1,11 @@
+// Shared timing constants. Keep magic numbers here so callers stay readable
+// and the rationale lives next to the value.
+
+// How long to wait for a Service Worker to answer GET_VERSION before
+// treating the response as missing. Short on purpose — SWs that take this
+// long are likely the Chromium zombie case (see pwa.svelte.ts).
+export const SW_VERSION_QUERY_TIMEOUT_MS = 1500;
+
+// Active update check while the page sits open in a tab. 30 minutes is a
+// trade-off between being timely and not hammering the server.
+export const SW_UPDATE_POLL_INTERVAL_MS = 30 * 60_000;