Add UI previews and NexaPG logo to README

Updated the README to include a centralized NexaPG logo and detailed UI previews of key application sections such as dashboard, targets management, query insights, and others. Added new screenshot assets to the `docs/screenshots` directory to support the visual updates.

README.md

@@ -1,5 +1,9 @@
 # NexaPG
 
+<p align="center">
+  <img src="frontend/public/nexapg-logo.svg" alt="NexaPG Logo" width="180" />
+</p>
+
 NexaPG is a full-stack PostgreSQL monitoring platform for multiple remote targets.
 It combines FastAPI, React, and PostgreSQL in a Docker Compose stack with RBAC, polling collectors, query insights, alerting, and target-owner email notifications.
 
@@ -19,6 +23,36 @@ It combines FastAPI, React, and PostgreSQL in a Docker Compose stack with RBAC,
 - SMTP settings in admin UI (send-only) with test mail support
 - Structured backend logs + audit logs
 
+## UI Preview
+
+### Dashboard
+
+![Dashboard Overview](docs/screenshots/dashboard-overview.png)
+
+### Targets Management
+
+![Targets Management](docs/screenshots/targets-management.png)
+
+### Query Insights
+
+![Query Insights](docs/screenshots/query-insights.png)
+
+### Alerts
+
+![Alerts](docs/screenshots/alerts.png)
+
+### Admin Settings
+
+![Admin Settings](docs/screenshots/admin-settings.png)
+
+### Target Detail (DBA Mode)
+
+![Target Detail DBA](docs/screenshots/target-detail-dba.png)
+
+### Target Detail (Easy Mode)
+
+![Target Detail Easy](docs/screenshots/target-detail-easy.png)
+
 ## Repository Layout
 
 - `backend/` FastAPI app, SQLAlchemy async models, Alembic migrations, collector services