feat(app,android,ios): search across episodes and people

A search now also finds, with posters and descriptions on, the directors
and studios and the genres whose names hold it, as chips that open their
page, and the episodes whose provider names hold it, which play from the
result (pstr_core::discover::find). The titles it found follow under
their own heading.

The last eight searches are kept in recent-searches.json on this device
only (pstr_core::recent), written only when a search led somewhere, and
offered under the empty search box with a way to clear them. The bridge
exposes search_extras, recent_searches, remember_search and
forget_searches.

On Android and iOS the library's titles are now the whole library
whatever is searched for; the grid narrows through the arrangement, as
it already did. A title opened from an episode result can then always be
shown, and History and Downloads no longer shrink to the search.

docs/PRIVACY.md

@@ -47,7 +47,7 @@ player is handed an opaque token, not a link, so no secret ends up in its log or
 
 | What | Where on Linux |
 |---|---|
-| Settings and the share list (without secrets) | `~/.config/proton-stream/` |
+| Settings, the share list (without secrets), My List and your last eight searches (`recent-searches.json`) | `~/.config/proton-stream/` |
 | The library and watch history (`catalog.db`) and downloads (`offline/`) | `~/.local/share/proton-stream/` |
 | Streamed video (`blocks/`), thumbnails and posters | `~/.cache/proton-stream/` |
 
@@ -62,6 +62,9 @@ can play them. Empty the cache in **Settings → Storage**, and delete downloads
 The library database holds file names and your watch history, in plain text. It holds no
 secrets.
 
+Recent searches are saved only when you open something a search found, are never synced, and
+**Clear recent searches** under the empty search box deletes them.
+
 ## Other people on the same computer
 
 - A share link given on the command line, as in `proton-stream 'https://…#key'`, is visible in