fix: make environment list accessible to all authenticated users

The list endpoint on EnvironmentAdminController now overrides the
class-level ADMIN guard with isAuthenticated(), so VIEWERs can see
the environment selector. The LayoutShell merges environments from
both the table and agent heartbeats, so the selector always shows
configured environments even when no agents are connected.

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

cameleer3-server-app/src/main/java/com/cameleer3/server/app/controller/EnvironmentAdminController.java

@@ -27,6 +27,7 @@ public class EnvironmentAdminController {
 
     @GetMapping
     @Operation(summary = "List all environments")
+    @PreAuthorize("isAuthenticated()")
     public ResponseEntity<List<Environment>> listEnvironments() {
         return ResponseEntity.ok(environmentService.listAll());
     }