feat(frontend): shared pod-wide test audio

Make the "Test audio" connectivity check pod-wide instead of local to the
publisher. Shared on/off state is derived from the presence of the podman-beat
track across participants (self-syncing on join/leave). Any participant can
stop it: the owner unpublishes directly; non-owners send a new additive
BEAT_STOP data message that the owner honors. Drives the button label, status
line, and status waveform for everyone, and guards against rapid-double-click
double-publish and mid-start unmount leaks.

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

shared/src/messages.ts

@@ -10,7 +10,9 @@ export type DataMessage =
   | { type: 'HERMES_MESSAGE'; message: HermesMessage }
   | { type: 'VOICE_CUE'; text: string }
   | { type: 'ACK'; interventionId: string; status: InterventionStatus; note?: string }
-  | { type: 'GIT_REPORT'; report: LocalGitReport };
+  | { type: 'GIT_REPORT'; report: LocalGitReport }
+  /** Any participant → the current test-audio owner: stop publishing the shared beat. */
+  | { type: 'BEAT_STOP' };
 
 /** A targeted teammate/project-channel notification from the Hermes action layer. */
 export interface HermesMessage {