Replace SaveWatchProgress (UPDATE watch_list_entry) with
UpsertContinueWatchingEntry so progress persists for any anime
without creating watchlist entries.
Previously the backend only fetched streams for the single requested
mode (defaulting to sub), so modeSources never had a dub entry.
The frontend's switchMode showed visual feedback but streamUrlForMode
returned an empty string since no dub token existed.
Now both modes are fetched and availableModes is derived from what
actually resolved.
