Add episodeCacheMode/count/days preferences (default: date, 60 days).
Apply the bound when reading instead of writing, so a preference change
takes effect without a refetch; the full parse cache stays intact so
fetch-more can page beyond the bound. Thread the window through
load/saveFeedsToFile and update tests and task docs.
Mirrors the Feed tab's '[Fetch More]' row inside a drilled show's episode
list (My Shows depth 1): shows only while the show's cache holds episodes
beyond its loaded window, and advances just that show's window by 50 on
Enter (or automatically at the bottom in auto mode). Same fetchMoreMode
preference drives both behaviors. Adds a store-contract test for the
per-feed pagination path.
