feat(filters): pick one year from a decade grid, in the feed and in Plex

A shared YearPicker: the recent years as chips with their counts, and a
More dialog that lays every year out by decade (sortable by count), so a
ninety-year library fits on one screen. The feed's scrolling popover gives
way to it; the Plex rail gets it above its range fields, a pick setting
both ends. The Plex facets now count titles per year, the year filter left
out.

backend/tests/test_plex_service.py

@@ -105,6 +105,28 @@ def test_the_facets_count_what_the_caller_would_see(db, make_user):
     assert (theirs["year_min"], mine["year_min"]) == (2001, 1990)
 
 
+def test_the_facets_count_titles_per_year_without_the_year_filter(db, make_user):
+    # The year picker lists every year with its count. Like each facet it drops its own filter (a
+    # picked year must not empty the list it was picked from) and keeps the others; movies and shows
+    # of one year add up, newest first.
+    me = make_user()
+    movies, shows = _lib(db), _lib(db, "2", kind="show")
+    _movie(db, movies, "a", genres=["Drama"], year=2001)
+    _movie(db, movies, "b", genres=["Drama"], year=2001)
+    _movie(db, movies, "c", genres=["Drama"], year=1990)
+    _movie(db, movies, "d", genres=["Horror"], year=1985)
+    _movie(db, movies, "e", genres=["Drama"])  # no year: not a year to pick
+    db.add(PlexShow(rating_key="s1", library_id=shows.id, title="S", genres=["Drama"], year=1990))
+    db.commit()
+
+    f = LibraryFilters(genres="Drama", year_min=2001, year_max=2001)
+    got = library.facets(db, me, scope="both", show="all", lang="en", f=f)
+    assert got["years"] == [{"year": 2001, "count": 2}, {"year": 1990, "count": 2}]
+    assert (got["year_min"], got["year_max"]) == (1990, 2001)
+    movie_only = library.facets(db, me, scope="movie", show="all", lang="en", f=f)
+    assert movie_only["years"] == [{"year": 2001, "count": 2}, {"year": 1990, "count": 1}]
+
+
 def test_an_episode_search_joins_the_callers_state(db, make_user):
     me, other = make_user(), make_user()
     lib = _lib(db, "2", kind="show")