fix(offline): surface degraded/missing preloads, join re-entrant runs, cache manifest to IDB

- loadManifest() goes through DataCache.fetchWithFallback: the file list is
  written to IDB and survives SW-cache eviction; a run with the reduced
  fallback set is flagged and shown as degraded (new i18n en/ru/cu).
- 404 answers are counted as 'missing' (deployment drift) with a
  preload-missing diag entry, instead of silently reporting Done.
- preload() is re-entrancy safe: concurrent calls join the in-flight run.
- versions.json is loaded once per session (was fetched per translation).
- Write-through failures in fetchWithFallback()/fetch() log idb-write-fail.
- preload-manifest.json SW route is NetworkFirst (fresh coverage online).
- Manifest now also covers the shared/lives + shared/commands fallbacks.

vite.config.ts

@@ -14,6 +14,16 @@ export default defineConfig({
         globIgnores: ['**/data/**'],
         maximumFileSizeToCacheInBytes: 20 * 1024 * 1024,
         runtimeCaching: [
+          {
+            // The preload file list must be fresh when online; a stale copy
+            // silently loses coverage. Offline falls back to the SW copy.
+            urlPattern: /\/data\/shared\/preload-manifest\.json$/,
+            handler: 'NetworkFirst',
+            options: {
+              cacheName: 'ponomar-manifest',
+              networkTimeoutSeconds: 3,
+            },
+          },
           {
             urlPattern: /\/data\/.*\.json$/,
             handler: 'StaleWhileRevalidate',