Add BYOC page, admin section, and is_byoc team visibility gating

- Frontend: BYOC hosts page (/dashboard/byoc) with register/delete flows,
  shimmer loading, pulsing online status, animated token reveal checkmark
- Frontend: Admin section (/admin/hosts) with platform + BYOC tabs, stat
  pills, skeleton loading, slide-in animations for new rows
- Frontend: AdminSidebar component with accent top bar and admin pill badge
- Frontend: BYOC nav item shown only when team.is_byoc is true (derived
  from teams store, not JWT); disabled for members
- Frontend: Admin shield button in Sidebar, visible only to platform admins
- Backend: is_admin in JWT claims + requireAdmin middleware (DB-validated)
- Backend: is_byoc added to teamResponse so frontend derives visibility
  from fresh team data rather than stale JWT fields
- Backend: SetBYOC admin endpoint (PUT /v1/admin/teams/{id}/byoc)
- Backend: Admin hosts list enriches BYOC entries with team_name
- Host agent: load .env file via godotenv on startup

internal/service/host.go

@@ -123,12 +123,15 @@ func (s *HostService) Create(ctx context.Context, p HostCreateParams) (HostCreat
 		}
 	}
 
-	// Validate team exists and is not deleted for BYOC hosts.
+	// Validate team exists, is not deleted, and has BYOC enabled.
 	if p.TeamID != "" {
 		team, err := s.DB.GetTeam(ctx, p.TeamID)
 		if err != nil || team.DeletedAt.Valid {
 			return HostCreateResult{}, fmt.Errorf("invalid request: team not found")
 		}
+		if !team.IsByoc {
+			return HostCreateResult{}, fmt.Errorf("forbidden: BYOC is not enabled for this team")
+		}
 	}
 
 	hostID := id.NewHostID()
@@ -370,9 +373,17 @@ func hashToken(token string) string {
 }
 
 // Heartbeat updates the last heartbeat timestamp for a host and transitions
-// any 'unreachable' host back to 'online'.
+// any 'unreachable' host back to 'online'. Returns a "host not found" error
+// (which becomes 404) if the host record no longer exists (e.g., was deleted).
 func (s *HostService) Heartbeat(ctx context.Context, hostID string) error {
-	return s.DB.UpdateHostHeartbeatAndStatus(ctx, hostID)
+	n, err := s.DB.UpdateHostHeartbeatAndStatus(ctx, hostID)
+	if err != nil {
+		return err
+	}
+	if n == 0 {
+		return fmt.Errorf("host not found")
+	}
+	return nil
 }
 
 // List returns hosts visible to the caller.
@@ -447,8 +458,8 @@ func (s *HostService) Delete(ctx context.Context, hostID, userID, teamID string,
 		return &HostHasSandboxesError{SandboxIDs: ids}
 	}
 
-	// Gracefully destroy running sandboxes on the host agent (best-effort).
-	if len(sandboxes) > 0 && host.Address.Valid && host.Address.String != "" {
+	// Gracefully destroy running sandboxes and terminate the agent (best-effort).
+	if host.Address.Valid && host.Address.String != "" {
 		agent, err := s.Pool.GetForHost(host)
 		if err == nil {
 			for _, sb := range sandboxes {
@@ -461,6 +472,10 @@ func (s *HostService) Delete(ctx context.Context, hostID, userID, teamID string,
 					}
 				}
 			}
+			// Tell the agent to shut itself down immediately.
+			if _, rpcErr := agent.Terminate(ctx, connect.NewRequest(&pb.TerminateRequest{})); rpcErr != nil {
+				slog.Warn("delete host: failed to send Terminate to agent", "host_id", hostID, "error", rpcErr)
+			}
 		}
 	}