feat(waybar): right-click volume opens a floating pwvucontrol mixer

Right-clicking the Waybar volume module now opens pwvucontrol (per-app
mixer); left-click still mutes. Adds:

- on-click-right in waybar.nix + all four whole-swap jsoncs (parity)
- hyprland.nix: the config's first windowrule block — float + center the
  mixer (pwvucontrol, with pavucontrol as a fallback class); normal-
  priority list so a downstream windowrule concatenates
- pwvucontrol in the template home.packages

pwvucontrol is PipeWire-native + GTK4/libadwaita, so it themes via the
portal color-scheme like Amberol. app-id verified from the built package.
This settles the windowrule pattern/placement; item 41 is now just
broadening the float set. Right-click→float→center is a V3 session check.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/downstream/home.nix

@@ -81,6 +81,7 @@
     inkscape            # vector graphics
     mpv                 # media player
     amberol             # music player (local library; streaming → spotify below)
+    pwvucontrol         # PipeWire volume mixer (right-click the Waybar volume)
 
     # ── More apps — uncomment to add ─────────────────────────────────
     # Web browsers