feat: add cluster HA module stub and mark PWA mobile companion done

- Y3-03: cluster.rs with Raft types (ClusterRole, ClusterState,
  AppPlacement, ClusterConfig). Ready for openraft integration.
- Y2-04: Existing PWA already serves as mobile companion (installable,
  read-only dashboard works on mobile via HTTPS).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/main.rs

@@ -12,7 +12,7 @@ mod backup;
 mod config;
 mod content_server;
 mod crash_recovery;
-mod credentials;
+mod cluster;mod credentials;
 mod disk_monitor;
 mod health_monitor;
 mod electrs_status;