Replaces /music-library.svg references in Sidebar and TitleBarNav with
/music-library-2.svg (the corrected all-black monochrome asset the user
provided) and swaps the <img> rendering for a <span> with CSS mask so the
icon inherits the theme's text color — same pattern now used for My QBZ,
cassette, collection, and the existing plex-mono icons.
cc
