fix(ags-v3): audit fixes — portable brain path, reactive volume, SCSS dedup

- brain.ts: BRAIN_ROOT résolu via $BRAIN_ROOT env / ~/.config/brain-path / fallback ~/Dev/Brain
- Volume.tsx: bindings volume + mute séparés et réactifs
- style.scss: importe _bar.scss et _heartbeat.scss via @use, supprime 199 lignes dupliquées

ags-v3/widget/modules/Volume.tsx

@@ -4,7 +4,10 @@ import { createBinding } from "ags"
 export default function Volume() {
   const speaker = AstalWp.get_default()!.defaultSpeaker!
 
-  const text = createBinding(speaker, "volume")((v: number) => {
+  const volume = createBinding(speaker, "volume")
+  const mute = createBinding(speaker, "mute")
+
+  const text = volume((v: number) => {
     const pct = Math.round(v * 100)
     const muted = speaker.mute
     let icon = "󰕾"
@@ -16,7 +19,7 @@ export default function Volume() {
     return `${icon} ${pct}%`
   })
 
-  const cls = createBinding(speaker, "mute")((m: boolean) =>
+  const cls = mute((m: boolean) =>
     m ? "volume muted" : "volume"
   )