Noctalia: muted headphones glyph must also be treated as mute in bar widget overrides

Context: noctalia-shell (C++), src/pipewire/audio_glyphs.cpp picks the volume icon; headphones output showed "headphones" but muted/0% fell back to speaker "volume-mute".

Fix: return "headphones-off" (Tabler ships it in assets/fonts/tabler.json, no alias needed) when muted or displayed volume 0% and headphones=true.

Gotcha: VolumeWidget::glyphName (src/shell/bar/widgets/volume_widget.cpp) detects mute by string-comparing the returned glyph to "volume-mute"/"microphone-mute". Any new mute glyph must be added there, otherwise user glyph/effects-profile overrides replace it and the mute override is skipped.

Testing: tests/audio_glyphs_test.cpp is not run by nix build .#default; compile it standalone: g++ -std=c++23 -Isrc tests/audio_glyphs_test.cpp src/pipewire/audio_glyphs.cpp.