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>

agent/BACKLOG.md

@@ -257,32 +257,16 @@ summer-day/summer-night) — all deliberately set `scrollbar { width:0 }`
 today, each needs a per-theme handle width/colour. Payoff is V3-only
 (rendering), so it wants a themed session, not a headless edit.
 
-### 35. Right-click volume → audio control app
-(Raised by Bernardo, 2026-07-07 — item 5.) Right-click the Waybar
-`pulseaudio` module to open a mixer in a floating window. Wire
-`on-click-right` in `waybar.nix` AND the whole-swap jsoncs (parity rule);
-add a Hyprland `windowrulev2 = float` for the mixer. Ship the mixer in
-the template's `home.packages`.
-_Scoped 2026-07-07 (held for item 41):_ the on-click-right wiring is
-trivial, but the float rule needs the windowrule infrastructure that
-**does not exist yet** — there are currently **zero** `windowrule`s in
-the whole config, and item 41 is where that set gets designed list-first
-(Bernardo's call). So this is gated on 41, not independently shippable
-without inverting his 41→35 plan. **Mixer recommendation:** `pwvucontrol`
-(PipeWire-native, GTK4/libadwaita → themes via the portal color-scheme
-like Amberol; the distro is already wpctl/PipeWire). app-id for the float
-rule: `com.saivert.pwvucontrol`. (`pavucontrol` is the GTK3 fallback.)
-
 ### 41. Floating-window audit
 (Raised by Bernardo, 2026-07-07 — item 11.) Decide which windows should
-open floating by default (config dialogs, mixer, file-chooser portals,
-calculators, etc.) and add the `windowrulev2 = float` rules in
-`hyprland.nix`. Enables items 35/36b. Produce the list first, then the
-rules.
-_Note 2026-07-07:_ this establishes the **first** windowrule block in the
-config (none exist today) — so it also owns the decision of where the
-rules live and their format. Once the pattern's set, 35's mixer float and
-36b fall out of it.
+open floating by default (config dialogs, file-chooser portals,
+calculators, etc.) and add the `windowrule` float rules in
+`hyprland.nix`. Enables item 36b. Produce the list first, then the rules.
+_Note 2026-07-07:_ the **first** `windowrule` block now exists (added by
+item 35 — the mixer floats + centers), so the pattern + placement are
+set. 41 is now purely "broaden the set" — audit the template app suite,
+pick the dialog/utility classes, append rules. Still list-first
+(Bernardo's taste on what should float), so it wants his review.