feat(feed): make episode cache bound user-configurable (date window or count)

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.

src/stores/app.ts

@@ -45,6 +45,9 @@ const defaultPreferences: UserPreferences = {
 	autoJumpToPlayer: true,
 	fetchMoreMode: "auto",
 	refreshIntervalMinutes: 30,
+	episodeCacheMode: "date",
+	episodeCacheCount: 25,
+	episodeCacheDays: 60,
 };
 
 const defaultState: AppState = {