v0.31.5: Library button edge to edge with the playlists; one floor; floating top row

The sidebar gets 8px of bare window on its left, and the Library button
spans exactly the playlist list below it, scrollbar included. The Library
button, search box, playlist name, times and playhead all stand on one
floor 5px above the lists (text by its baseline). The top row floats:
equal space before, between and after the controls, with the now-playing
panel a fixed share of the width (700px at 1920). The header row grows if
the app font makes the Library button taller than the strip.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_round17.py

@@ -278,8 +278,6 @@ class TestSearchBarPlacement:
         manager = _manager(tmp_path, [Track(track_id=1, name="A")])
         view = LibraryView(manager)
         assert view._search.parentWidget() is view._top_strip
-        view.set_header_height(34)
-        assert view._top_strip.height() >= 0  # metrics path still works
 
     def test_search_still_filters(self, qapp, tmp_path):
         from lintunes.gui.library_view import LibraryView