feat: add post-download prompt with open book / back to listing options

After an OPDS download completes, show a prompt screen instead of
immediately returning to the catalog. The user can choose to open the
book for reading or go back to the listing. A live countdown (5s, fast
refresh) auto-selects the configured default; any button press cancels
the timer. A per-server "After Download" setting controls the default
action (back to listing for backward compatibility).

Made-with: Cursor

lib/I18n/I18nKeys.h

@@ -427,6 +427,10 @@ enum class StrId : uint16_t {
   STR_SELECT_FOLDER,
   STR_DOWNLOAD_PATH,
   STR_POSITION,
+  STR_DOWNLOAD_COMPLETE,
+  STR_OPEN_BOOK,
+  STR_BACK_TO_LISTING,
+  STR_AFTER_DOWNLOAD,
   // Sentinel - must be last
   _COUNT
 };