feat(search): episode search scope with tab toggle

Search now covers individual episodes, not just shows: the iTunes
Search API (entity=podcastEpisode) matches episode titles and show
notes, so a guest or topic finds the episodes they appear in across
shows. Enter on an episode result subscribes to the parent show.

- types: SearchResult becomes a kind-discriminated union
  (podcast | episode); EpisodeSearchResult carries the parent show so
  existing consumers compile unchanged
- source-searcher: searchEpisodesByType (RSS/CUSTOM return []),
  buildItunesEpisodeUrl, cleanDescription (HTML -> text),
  mapItunesEpisodeResult (episode id/duration ms->s/audioUrl, reuses
  mapItunesResult so delisted shows keep a directoryUrl)
- search: searchEpisodes with an 'episode' cache/dedupe namespace so
  shows and episodes for the same query never mix
- stores/search: scope signal (podcast | episode) persisted to
  podtui_search_scope; search() branches on scope
- SearchPage: Shows/Episodes pills row with 'tab to toggle', scope-
  aware placeholder/empty state/result rows (episode row = title +
  Show · date) and preview; toggling re-runs the current query
- keybinds: search-scope-toggle bound to tab (keybinds.jsonc AND the
  runtime DEFAULT_KEYBINDS merge so the binding exists for users with
  a pre-existing config file); while the input is focused the Shell
  router never sees Tab, so the input handles it via onKeyDown +
  preventDefault (no double-toggle: the router path only fires when
  the input is defocused)
- Shell help overlay documents [tab] shows/episodes

src/components/Shell.tsx

@@ -447,6 +447,7 @@ function helpSections(k: ReturnType<typeof useKeybinds>) {
 				["enter", "open"],
 				["r", "refresh"],
 				["s", "search"],
+				[p("search-scope-toggle"), "shows/episodes"],
 				["f", "filter"],
 				[",", "sort"],
 				[".", "hidden"],