Backend retention now keeps only recent posts, making the time filter UI
and newerThan filtering obsolete. Deleted use-time-filter hook, canonical
multiboard routes only, removed footer suggestions, dropped i18n keys.
Added `useFileUpload()` and `uploadToCatbox()` to centralize upload behavior and remove duplicated upload code from `post-form.tsx` and `reply-modal.tsx`. The flow now routes Android to `FileUploader`, Electron to hidden-input + catbox upload, and web to a translated fallback alert with consistent upload UI state.