refactor: item 29 cleanup

F16 F17 F18 F19 F20 F21: import polling backs off and pauses in a hidden tab, one date format module, fake timers in the import tests, error codes instead of raw messages, one safe storage wrapper, dead code.

README.md

@@ -144,6 +144,10 @@ flowchart LR
 - `src/utils/brokenImages.js` reports pictures that fail to load (`GameCover`, achievement icons on the game
   detail page) to Core.
 - `useLibrarySync` polls Core's `/api/library/sync-status` while a Steam sync runs, then loads the data again.
+- The import page polls a running import every 2 s. After a `5xx` or a network error it waits twice as long (up to
+  30 s) and gives up after 10 failures in a row; a hidden tab does not poll.
+- All `localStorage` access goes through `src/utils/storage.js`: with site data blocked a read is `null` and a
+  write is lost, but the app still starts.
 - `PUT /api/users/settings` replaces dark mode, language, display name and public profile at once, so every save
   goes through `saveSettings` in `src/utils/settings.js`: one after the other, each on a fresh `GET /api/users/me`.
 - Pages keep a load error as its i18n key and translate it when rendering, so a language switch loads nothing
@@ -211,7 +215,7 @@ npm run build
 
 ## Roadmap
 
-See the roadmap in the ToBePlayed-Core readme.
+See `ROADMAP.md` in the ToBePlayed-Deployment repo.
 
 ## License