feat: change default listen port from 3000 to 1852 (#756)

Updates the backend listen port, Vite dev proxy target, Docker EXPOSE,
compose port mapping, .env.example default, GitHub Actions smoke-test
default, healthcheck URLs, and every doc/example reference. Test fixtures
that include example URLs were updated for consistency, though their
assertions are port-agnostic.

The rate-limit value of 3000 in middleware/rateLimiters.ts and the
3000 entry in WEB_UI_PORTS (which detects user containers like Grafana)
are intentionally untouched.

backend/src/__tests__/users-rbac.test.ts

@@ -568,7 +568,7 @@ describe('Orphaned role assignment cleanup', () => {
   it('deleting a node removes its role assignments', async () => {
     const db = DatabaseService.getInstance();
     // Create a test node
-    const nodeId = db.addNode({ name: 'test-cleanup-node', type: 'remote', api_url: 'http://test:3000', api_token: '', compose_dir: '/tmp', is_default: false });
+    const nodeId = db.addNode({ name: 'test-cleanup-node', type: 'remote', api_url: 'http://test:1852', api_token: '', compose_dir: '/tmp', is_default: false });
     // Create a role assignment for this node
     const hash = await bcrypt.hash('password123', 1);
     const userId = db.addUser({ username: 'nodeorphan', password_hash: hash, role: 'viewer' });